We investigated the influence of lapping, polishing and chemical etching of semi-insulating CdZnTe by the contactless resistivity and photoconductivity method. This method can determine the sample parameters independent of the type and quality of the metallization. We observed that the evaluated sample resistivity varies with the surface preparation method up to a factor of two. The photoconductivity anti-correlates with resistivity and it changes strongly within one order of magnitude. We determined a correlation between surface roughness, oxide layer thickness and material resistivity. Deviation of the trends is visible with surface preparation by chemical etching. We propose an optimal surface treatment to maximize the resistivity and thus to decrease the dark current. (C) 2014 Elsevier B.V. All rights reserved.