Can a merger from duopoly to monopoly be detrimental for profits? This paper deals with this issue by focusing on the interaction between decreasing returns to labour (which imply firms’ convex costs) and centralized unionization. First, it is highlighted that a wage ‘non-rigidity’ result applies: the post-merger wage is higher than in the pre-merger equilibrium. Second, it is shown that a ‘reversal result’ in relation to merger profitability actually realizes when the union is sufficiently oriented towards wages. Moreover, the higher the reservation wage, the degree of product differentiation, and the union's relative bargaining power, the higher the probability that a merger reduces profits.