This study investigates the underlying constructs of two instruments which purport to measure the locus of control of reinforcement variable. Instruments used were the IAR and the Career Development Responsibility Scale (CDR). Principal components analyses were computed separately for each instrument from the intercorrelation matrix of the items of the respective instruments. Data were obtained from a cross-sectional student group of ninth graders. The study provides evidence that instruments designed to measure locus of control in domains other than academic achievement serve to increase the number of dimensions. Intercorrelations of the factor score derived from the IAR and CDR are sufficiently low as to question whether or not they are, in fact, measuring the same construct. Developers of locus of control instruments should be aware that the balance of situations and sources of control items will affect the scores obtained from the instrument, and thus the relationship with other similar instruments. (Author/PC)