Biodiesel is produced by methanolysis or ethanolysis of triglycerides, and there are many factors affecting the transesterification process. The most important variables of the transesterification reaction are as follows: quality of the oil, catalyst type, concentration of catalyst, molar ratio of alcohol to oil, temperature and reaction time. The literature studies show that there is not a generally accepted procedure for the characterisation of transesterification reactions results and catalyst formulation activity. Therefore, even the evaluation of the usability of the most utilised industrial catalysts (sodium hydroxide and potassium hydroxide) is inconsistent. The experimental investigations of the transesterification reactions of high quality rapeseed oil in the presence of the above mentioned catalysts by change of all the variables allow us to regard two different characteristics of each reaction (reaction yield and process yield) that has to be determined. The comparison of these two characteristics gives a new instrument for classification of catalyst formulations. In the case of high quality rapeseed oil, sodium hydroxide and potassium hydroxide have shown a similar activity. Sodium hydroxide is more preferable when the concentration area of the used catalysts is low, but potassium hydroxide – at the high concentration area of catalysts.