Dehydrogenase activity (DhA) in soils was determined by the Lenhard method using the original procedure of Casida et al. (1964) and the modification of this procedure with the use of Na2CO3, or Ca(OH)2 instead of CaCO3, and also by the Thalman method. Also pH of soil suspensions was tested. The lowest values of DhA were obtained by the Thalman method, higher by the Lenhard method. The lowest concentrations of formazan were obtained with the use of CaCO3, intermediate with the use of Na2CO3, and the highest concentrations with the use of Ca(OH)2. In most cases the determined values of DhA in tested soils were not proportional between respective methods. The use of CaCO3 or Na2CO3, as well as of a buffer, in the Thalman method, did not assure the optimum pH for dehydrogenases. After sterilization of tested soils and incubation with TTC and Ca(OH)2 the obtained formazan concentration was close to that of non-sterilized soils. After sterilization of the sandy soil and incubation with TTC and Na2CO3 the obtained formazan concentration was on the level of 26% in relation to non-sterilized soil. The causes for diversified DhA in tested samples are discussed in the paper. It was concluded that pH of the soil designated for DhA determination should not be regulated and the control samples should be prepared from a sterilized soil with addition of TTC, taking into account the possibility of chemical reduction of TTC.