We show that the transition in AMnO3 from the orthorhombic perovskite phase to the hexagonal phase is promoted by inducing site disorder on the A-site. The gap between the orthorhombic and the hexagonal phase is widened for site disordered, mixed yttrium−gadolinium manganite samples. At the cost of the orthorhombic phase, a two phase region emerges. The phase separation exhibits very unusual thermodynamical behavior. We also show that high-pressure synthesis favors the orthorhombic phase. YMnO3 is formed in the orthorhombic phase at 15 kbar.