Described is a method to prepare wet foams exhibiting long-term stability wherein colloidal particles are used to stabilize the gas-liquid interface, said particles being initially inherently partially lyophobic particles or partially lyophobized particles having mean particle sizes from 1 nm to 20 .mu.m. In one aspect, the partially lyophobized particles are prepared in-situ by treating initially hydrophilic particles with amphiphilic molecules of specific solubility in the liquid phase of the suspension.