There is substantial empirical evidence showing the impact of the type of numbers in the solution of problems with a multiplicative structure. The theory of the intuitive models of arithmetic operations is today the most plausible theoretical account for these findings. In an attempt to contribute to a better understanding of the effects of number type, a study was carried out in which student-generated word problems were used as data to test a series of predictions derived from the theory of the intuitive models. Subjects were 107 12-year-olds, 107 15-year-olds, and 99 elementary student teachers in Belgium. Results concerning the influence of the type of number sentence on performance are reviewed and the congruence of the generated problems with the primitive models are discussed. Several findings of this study support the theory of Fischbein et al. (1985) that the selection of an operation for solving multiplication problems is mediated by primitive intuitive models of the arithmetic operation. (Author/MKR)