Frequency scanned radiometric measurements from rough absorbers are different from smooth samples. This effect has to be considered for accurate data interpretation, for instance, for photothermal depth profiling. Based on the equivalent layer model we discussed the effect of roughness induced thermal wave dispersion and derived a formula which correctly describes the observed experimental features: (i) appearance of phase difference extremum; (ii) rise of the relative signal magnitude at increased frequencies. The validity of the theory was successfully demonstrated by comparing calculations with radiometric measurements from rough steel samples. (C) 2002 Elsevier Science B.V. All rights reserved.