In this paper we describe the validation of two scales constructed to measure pre-university students' changing disposition (i) to enter Higher Education (HE) and (ii) to further study mathematically-demanding subjects. Items were selected drawing on interview data, and on a model of disposition as socially- as well as self- attributed. Rasch analyses showed that the two scales each produce robust one-dimensional measures on what we call a 'strength of commitment to enter HE' and 'disposition to study mathematically-demanding subjects further' respectively. However, the former scale was initially found to suffer psychometrically from a ceiling effect, which we 'corrected' by adding some harder items at a later data point, and revised the scale according to our interpretation of subsequent results. We finally discuss the potential significance of the constructed measures of learning outcomes, as variables in monitoring or even explaining students' progress into different subjects in HE.