Parliamentarians are often allowed to pursue other work in addition to their mandate. Using data on the 7th European Parliament (2009–14), we analyze the relationship between the outside earnings of its Members (MEPs) and their parliamentary activities. The supranational nature of the European Parliament thereby allows a novel analysis of ‘moonlighting’ free of country‐specific bias. We find outside earnings to be negatively correlated with the particularly work‐intensive production of draft reports and opinions. Utilizing the considerable freedom of Member States in organizing elections, we find this relationship to be dependent on the electoral system under which MEPs are elected. While the effect of the trade‐off between outside and parliamentary work is predominantly negative in all other systems, outside earnings of MEPs from centralized but candidate‐focused systems correlate positively with their productivity, indicating a possible benefit from selection effects.