The institutions of the European Union (EU) pass legislation which affects 508 million inhabitants (Europa 2017a), 26 million enterprises, 143 million business economy workers, and the national policies of 28 Member States (Eurostat 2017). To date the EU has passed over 40,000 legal acts. Codecision refers to a legislative procedure in operation in the European Union decision-making process. Since 1999, informal tripartite meetings called “trilogues” between the EP, European Commission [Commission] and the Council have taken place prior to and separate from the main public “reading stages” of codecision legislation (where the EP and Council enjoy equal decision-making powers) so as to increase the efficiency of decision-making. A trilogue is an informal meeting or negotiation between representatives of the Commission, the EP and the Council. Commentators, including the EU Ombudsman, have questioned the democratic legitimacy of these “trilogue” meetings, numbering 1500 over the past five years (EU Ombudsman 2016), in the context of the EU’s democratic deficit and the need for decision-making transparency. Using logistic regression models, it is the aim of this present study to (a) investigate when and under which conditions trilogues occur and, (b) separately, but keeping in line with the charge against democratic legitimacy, to explore the conditions which potentially affect the transparency of legislative files. This will include the implications for transparency of efficiency and budgetary considerations. Parts (a) and (b) will be aided by a broad database consisting of 1448 Ordinary Legislative Procedure (OLP)-agreed directives, regulations and decisions. The findings in (a) and (b) will be illustrated further with the use of qualitative case studies of legislative files and interviews with MEPs and an EP legal advisor. Focusing on three full EP legislative terms and part of the subsequent one, this study is unique in its scope. It explores legislation at all reading stages in pursuit of understanding why files are trilogued. Equally novel is the examination of the same 1448 OLP codecision files to determine the transparency of both trilogued and non-trilogued legislative processes, in order to investigate concerns about the democratic credentials of trilogues. The study finds that files that are trilogued are usually building types where sovereignty, economic efficiency and budgetary implications matter. In the post-declaration period (2007-2016), the transparency of legislative file documentation is not adversely affected by efficiency in the time taken to conclude files. However, EU community budgetary implications matter. Increases in budgetary amounts see a corresponding reduction in the transparency of legislative file documentation.