Stratospheric ozone depletion was first reported in 1985. Early on, researchers identified polar stratospheric clouds (PSCs) as being important in chemistry related to ozone depletion. PSCs exist as crystalline water-ice particles (type II), and as crystalline (type Ia) or liquid (type Ib) particles stable above the water-ice frost point. Uncertainty remains concerning the composition and formation mechanism of the most common PSC, type Ia. Here, we consider likely formation mechanisms for type Ia PSCs.