The failure strengths of soda‐lime glass rods exposed to 9 liquids with polarities from 30.9 to 63.1 kcal/mol were determined experimentally in flexure. The fracture strength clearly decreased with increasing polarity. The necessity of maintaining a high degree of purity in the environmental fluids to obtain meaningful results was illustrated by tests in which controlled amounts of a highly polar liquid (methanol) were introduced into puren‐hexane. Traces as small as 0.004% reduced the strength significantly whereas a 0.05% addition reduced it to that in pure metha