The aim of research was validation of a method for the assessment of cognitive complexity of chemical technology problem tasks. The method included an assessment of the difficulty of concepts and an assessment of their interactivity. As a research instrument for assessing performance, test of knowledge was used. Each task in the test was followed by a 5-point Likert scale for the evaluation of invested mental effort. The validity of this method was confirmed by a series of linear regression analysis where extremely high values of correlation coefficients are obtained among the examined variables: student's performance, invested mental effort and cognitive complexity. Keywords: cognitive complexity, problem solving, chemical technology. [For the full proceedings, see ED619611.]