Committee assignments in parliamentary democracies can be modeled as an intra-party delegation relation, in which party leaders (as principals) select legislators (as agents) to represent the party on specific committees. Standard models of delegation would lead us to expect that the assignment process is systematically influenced by the legislators’ reputation for loyalty, their expertise and their potential contribution to the party’s collective goals. In this paper, we make use of an original dataset to conduct a longitudinal analysis of committee assignments in the United Kingdom, from 2001 through 2015. We propose a delegation model of committee assignments in which the party leaders assign legislators according to the legislators’ own preferences and expertise as well as their loyalty to the party in the recent past. We measure legislators’ preferences for, and expertise in, a specific policy area by examining the parliamentary questions tabled in previous legislative terms. Also, we use information on the MPs’ behavior in the division lobby to establish their loyalty incentivizing the leadership to accommodate the legislator’s preferences.