In 1965, Belgium suffered the longest political crisis in its history, the first in a long series. It was sparked by the voting through, under the previous parliament, of a declaration to revise the Constitution, at a time when views on language and language Community issues were becoming more polarised. The crisis resulted in recourse to unusual negotiating methods, and was a precursor of the institutional and partisan developments which would change the country for ever. This article aims to understand the real role played by the revision of the Constitution in this crisis. To do so, we will examine the role of the various protagonists and their positions. We shall also highlight the solutions proposed to the conflict between the language Communities and the negotiation processes used, as well as tracting various chronological stages in the development of the crisis. We will try, when it is relevant, to include this approach in Lipset and Rokkan's cleavage theory, which applies particularly well to Belgian politics in the 1960s. The specific circumstances, however, will aso be examined within a broader timeframe, in ordor to analyze the crisis as an expression of a set of latent developments. This research approach required the use of unpublished archives, such as the archives of the informateur, of the formateur, and of the party presidents. We consulted the private party archives of PSC-CVP, PSB-BSP and PLP-PVV, as well as the press and published sources.