Powder samples of Fe_1-xCu_xCr₂S₄ with x = 0,0.2,0.5,0.8 were studied, between 5 and 300 K. The results reveal that for x < 1, the magnetic order in the series is more varied than the simple collinear ferrimagnetic structure traditionally assumed to exist everywhere from the Curie point to T → 0. In FeCr₂S₄ several ordered magnetic phases are present, with the ground state likely to have an incommensurate cone-like helical structure. Fe_0.8Cu _0.2Cr₂S₄ is the compound for which simple collinear ferrimagnetism is best developed. In Fe_0.5Cu _0.5Cr₂S₄ the ferrimagnetic spin structure is not stable, causing spin reorientation around 90 K. In Fe_0.2Cu _0.8Cr₂S₄ the ferrimagnetic structure is at low temperatures considerably distorted locally, but with rising temperature this disorder shows a rapid reduction, coupled to increased spin fluctuation rates. In summary, the present data show that the changes induced by the replacement of Fe by Cu have more profound influences on the magnetic properties of the Fe_1-xCu_xCr₂S₄ compounds than merely a shift of Curie temperature, saturation magnetization and internal field magnitude.