The main objective of this work was to study the modification introduced in the functionality of modified activated carbons. Carboxylic acids, lactones, anhydrides, phenolic groups, carbonyl groups and quinones can be analyzed by temperature programmed desorption experiments with mass spectrometric detection (TPD) [1]. Activated carbon obtained by physical chemical activation method from nut shells (CAN) has been modified with chlorine aqueous solution (CAN-Cl) to introduce chlorine surface groups. The TPD has been used to study the surface functionality of the active carbons. For TPD analysis 10 mg of sample were placed in quartz-molybdenum tube and the tube was evacuated to 10-2 Pa and then heated at 10oC/min up to 800 oC. The gases produced along the decomposition of surfaces groups were analyzed with a mass spectrometer. The CO2 released from CAN and CAN-Cl samples at temperatures lower than 400°C indicates the presence of carboxylic groups on active carbons surface. CO released from CAN and CAN-Cl active carbons beyond 600°C shows the presence of phenolic groups, carbonyl groups and quinones. The species evolved for chlorine groups were mainly mass 36 (HCl35); this means that the groups decompose mainly to HCl. Also, release of CH4 and C2H6 from active carbons can be explained by uncompleted (partial) activation of charcoal.