Simulation results of previous authors show an ambiguous effect of increased price flexibility on output stability in models incorporating a Mundell inflation effect on aggregate demand. This paper interprets their results in an analytically tractable model with imperfect, goods-market competition. To be destabilizing, increased flexibility must increase the “hump” of the price level's response to demand shocks. Output variability is always reduced by increasing the size of the flex-price sector and sometimes reduced by shortening contract lengths in the fix-price sector.