This study assessed the functioning and measurement properties of Rotter's Locus of Control Scale in its Spanish version by using a new approach based on Item Response Theory. Our procedure allows us: (a) to detect the impact of response biases (acquiescence and social desirability); (b) to evaluate clearly the test dimensionality and structure; (c) to evaluate and understand the causes of some items' inefficiency; and (d) to increase measurement precision. Globally, the results do not contradict those obtained in previous research, but allow some of the criticisms the scale had received to be qualified. Some recommendations for use are proposed.