AbstractCrosslinked poly(styrenesulfonic acid) beads in contact with AlCl3vapors at 115°C reacted to give HCl and a complex incorporating S, Al, and Cl in the ratio 2:1:2. Electron microprobe x‐ray analysis showed that the complexes were distributed uniformly throughout the polymer structure, which consisted of 200 Å microspheres surrounded by a labyrinth of pores. The polymer is a very strong proton donor, comparable to the superacid solution SbF5+ FSO3H. Its structural and acidic properties are inferred to be similar to those of the complex formed from AlCl3and H2