Abstract: Polymer particles each containing at least one void and also a hydrophobic copolymeric surfactant and a hydrophilic copolymeric surfactant. The hydrophobic surfactant comprises long hydrophobic chains anchored in the particles and hydrophilic chains exposed to the voids. The hydrophilic copolymeric surfactant comprises hydrophobic portions anchored in the particles and hydrophilic chains exposed to the surroundings of the particles. The particles have a fine cell structure, are washable to remove unanchored surfactant and are easily dispersible in water. They are useful in coatings, plastics, cosmetics, paper and leather. Also a process for making the particles in which water is dispersed as droplets in monomer containing the hydrophobic surfactant and the dispersion is in turn dispersed in additional water containing the hydrophilic surfactant and the whole is subjected to a polymerisation which forms particles containing voids initially filled with water.