When polyacrylamide gel is prepared from a mixture of acrylamide, N,N'-methylene-bisacrylamide and triethanolamine with a peroxodisulfate initiator in the presence of sulfide, spatial pattern formation occurs in the gel if water is spread on top of the gel after it solidifies. These patterns, typically arrays of spots or stripes, persist for many hours. Analysis of the effects of temperature, composition, stirring, and other factors suggests that the pattern formation occurs during polymerization due to convection driven by the highly exothermic polymerization reaction. The role of sulfide ion, which is essential for the pattern formation, appears to be to terminate the polymer chain elongation while maintaining the overall exothermic chain reaction.