Theory: The adoption of EC secondary legislation can be analyzed from the perspective of agency theory whereby Member States and the Parliament delegate policy authority to the Commission and design ex-post control procedures (i.e. Comitology). Rational choice and sociological institutionalisms differ in their predictions on the way rules and norms affect the extent of executive discretion. Hypothesis: Three institutionalist hypotheses are tested. The rationalist one derives from a Bayesian game developed by the author. It posits that Commission s executive discretion in non amending secondary legislation is a function of: 1) formal legislative procedure, 2) information asymmetry and 3) distribution of principals preferences. A fourth variable, legislative instrument, is also included. The diluted rationalist hypothesis substitutes formal with informal procedure in one policy area. The socio-rational hypothesis adds two new variables, that is the opinions of the Parliament and the Economic and Social Committee. A final co-graduation test is conducted on whether more discretion leads to more stringent ex-post control. Methods: Given the bimodal error structure of the regression model, I have bootstrapped the regression coefficients and computed the 95% confidence intervals of the null hypothesis. Bootstrapping has also been used to test the role of the European Parliament, of opinions and the co-graduation between discretion and ex-post control. A stratified sample of non amending secondary legislation adopted from 1987 to 1993 has been drawn to test the hypotheses. Results: The diluted rationalist hypothesis is the most accurate. Information asymmetry, informal legislative procedures and legislative instruments are statistically and substantively relevant in explaining executive discretion. Distribution of preferences has weak explanatory power probably because of the lack of reliable data and appropriate measurement. The Parliament and opinions do not relevantly affect Commission s discretion. More discretion leads to more confining ex-post control.