In many proportional representation (PR) systems, a legal electoral threshold has been implemented. Such a threshold reserves the allocation of seats to those parties which reach a minimum number of votes or a percentage of the votes. Since 2004, a legal threshold of five per cent has been introduced for the Belgian regional elections, following the introduction two years earlier of the same mechanism for the federal elections. The main reason behind this electoral reform – in a country which has been quite stable on this regard – was to prevent the further fragmentation of the political spectrum. This paper aims at testing this claim on empirical grounds. Two regional elections – 2004 and 2009 – can be surveyed in order to measure the effects of the electoral threshold. It is particularly relevant to study the impact of the electoral threshold in the regional elections because there are several discrepancies between the Regions in terms of the size of the districts, the fragmentation of the party system and the electorates. These differences may indeed influence the potential effects of the legal threshold. Since Duverger’s seminal work, electoral rules are known to influence the party system through their impact on the electoral outcomes, on the one hand, and on the electoral behaviour of both parties and voters, on the other hand. Two effects, which have been much further developed by the subsequent literature and are now widely accepted, are at stake here: the mechanical and the psychological. While the former refers to the objective mechanism of under-representation of some – usually the smaller – parties which may potentially lead to their disappearance because of the electoral laws, the latter implies the more subjective mechanism which can play at the level of both voters and party elites. Thus, in order to measure the impact of the legal five per cent threshold on the fragmentation, this paper tests the mechanical and the psychological effects for the Belgian regional elections of 2004 and 2009.