In the present study, within the framework of a single model we analyze transport and optical phenomena in narrow-gap semiconductor PbSnTe:In in relation with the presence of traps in the material. At helium temperatures, the narrow-gap semiconductor Pb1-xSnxTe:In with x≈0.24-0.28 (Eg≈0.060.04 eV) behaves as an insulator [1]. At Т=4.2 K, the resistivity of the material can exceed 108 Ohm⋅cm, this value translating into a resistance per square value of thin-film samples of micrometer thickness of 1012 Ohm/□. The mobility of charge carriers in PbSnTe:In can reach values of 105 cm2⋅V-1⋅s-1, those values being 104-105 times greater than the mobility of charge carriers in ordinary insulators. Together with the high value of static dielectric permittivity (up to 3000-4000 for the indicated range of material compositions), the latter leads to a situation in which space-chargelimited currents typical of insulators become substantial in PbSnTe:In even in weak electric fields and at inter-electrode distances of order several millimeters. A specific feature of space-charge-limited currents in insulators is their strong dependence on trap parameters, in particular, on the energy spectrum of traps, which in PbSnTe:In can be rather intricate [2]. The latter is manifested both in the stationary current-voltage characteristics of samples and in transient currents, including current oscillations observed in non-illuminated samples [3]. The energy position of electron traps corresponds to submillimeter spectral region. That is why it is in that spectral region where many interesting phenomena due to the complex energy spectrum of electron traps in PbSnTe:In are manifested. Using free electron laser (FEL) radiation, we observed a resonant behavior of the Hall coefficient with increasing the magnetic field strength in samples exposed simultaneously to band-to-band and submillimeter illumination. The position of peaks in the current vs magnetic field curves was found to be dependent on FEL radiation wavelength; this dependence can be explained by matching of photon energy hνFEL to the position of trap levels in the forbidden band of PbSnTe:In during their continuous «motion» in the bandgap with increasing the magnetic field value (magnetic freezing-out phenomenon) [4]. In the present study, the injection currents in PbSnTe:In films of micrometer thickness flowed along the surface. With the film thickness being much smaller than the separation between the contacts, the surface has a strong effect on the electric current. For instance, the angular dependences of the electric current in strong (up to 4 T) magnetic fields during rotation of the field through 360° turned out to differ strongly for orientations for which moving charge carriers were deflected by the magnetic field towards the substrate or towards the free surface of PbSnTe:In [5]. An important feature here is that, on changing the magnetic-field direction, the current changed its magnitude quite rapidly (with characteristic times of 0.1 sec or shorter) in comparison with typical durations of trap-induced phenomena in PbSnTe:In (seconds, hours, or longer times). A model implying scattering of charge carriers by the free surface of PbSnTe:In films was considered. [1] B.A. Akimov et.al. JETP Lett. 29 (1979) 9. [2] A.E . Klimov and V. N. Shumsky. Physica B 404 (2009) 5028. [3] A.N. Akimov et.al. Sensor electronics and Microsystems technologies 8 (2011) 20. [4] A.E. Klimov, V.V. Kubarev, N.S. Paschin, and V.N. Shumsky. Proc. 3rd Int. Conf. on Fundamental Problems of Micro- and Nanosystems Technology, Novosibirsk, Russia, December 1-4 (2011) 3. [5] A. Klimov, V. Shumsky, and V. Epov. Proc. Symp. on Surface Science 2012, St. Christoph am Arlberg, Austria, March 11 – 17 (2012) 75.