Reported is a study using tasks described by Piaget and Inhelder to evaluate changes in reasoning ability. The tasks were to evaluate the development of interpropositional logic, i.e., formal operations. The tasks used were validated using a formal operational population. They were administered at the beginning and end of the school year to eighth- and ninth-grade learners enrolled in science courses using newly developed science curricula. Control groups taking general science were evaluated at each grade level. Analysis of covariance was used to control differences between groups. The study showed that logical thinking processes can be evaluated using the Piagetian tasks. It was found that some curricula were effective at specific grade levels. The Time, Space and Matter curriculum was significantly more effective than were either the Introductory Physical Science (IPS) or Earth Science Curriculum Project (ESCP) materials. IPS was significantly better than the ESCP curriculum. At the ninth-grade level, both ESCP and IPS were significantly better than the control, and the IPS curriculum was significantly better than the ESCP curriculum. It was found that sex and IQ were not significantly related to gain in logical thinking ability. (Author/EB)