In two experiments, a modified air-comparison procedure was employed in two experiments to establish and assess membership functions for numerous probability terms. In both cases, subjects judged: a) to what degree one probability term better described that probability than another, and b) to what degree one term rather than another better described a probability. Task a) data from subjects was analyzed in terms of the axioms of an algebraic-difference structure, and membership function values were obtained for each term according to various ratio and difference scaling models. The axioms were well satisfied, and goodness-of-fit measures for the scaling procedures were quite high. Furthermore, the derived membership functions had interpretable shapes and satisfactorily predicted for each subject the judgements independently obtained in b). These results support the claims that the scaled values indeed represented the vague meanings of the terms to the subjects in the present context. Keywords: Cognitive psychology, Probability theory.