The main problem in students' lower achievement lies in the cognitive complexity of the problem. The aim of this research was to create and validate the procedure for the assessment of the cognitive complexity of chemical technology problem tasks. The procedure included the creation of Tables for assessing the difficulty of concepts in chemical technology problems and their interactivity, assessment of the numerical rating of cognitive complexity of the analyzed tasks, and conducting of research. Research included 50 students. Data were collected with the test of knowledge which was used for the assessment of students´ achievements and invested mental effort. The validity of this procedure was confirmed by a series of correlation analyses where statistically significant values of correlation coefficients were obtained among the examined variables: students' achievements, invested mental effort and cognitive complexity. The largest contribution of this procedure is that it is designed to show an objective value of the cognitive complexity of tasks in the domain of chemical technology. Good estimation of the numerical values of cognitive complexity can help teachers to better predict students' achievement, and at the same time to take care to avoid cognitive load.