Presented is a model of problem solving that incorporates elements of hypothetico-deductive reasoning in the Piagetian sense, and heuristic-algorithmic processing in the information-processing sense. It assumes that people invoke both formal reasoning strategies and learned algorithms whenever they solve problems. The proposed model integrates the Piagetian model of formal operational thought, and Polya's problem-solving heuristic model, together with Newell and Simon's General Problem Solver algorithm. A number of subroutines have been added to this integration in order to increase its scope. To test the validity of the model, an experimental design was set up with three variables: subjects, tasks, and conditions. Three subject groups (15 and 20 year olds), two comparable tasks which bore an inverse relationship to each other, and plan and do conditions were used. The interpretation of the results of the experiment supports the original assumption. The new model appears to be useful in removing ambiguity in the two mentioned modes of reasoning. (GA)