A catalyst enables hydrosilylation of aliphatically unsaturated carbon-carbon bonds by silicon-bonded hydrogen to be conducted rapidly in water. The catalyst provides a rapid method of carrying out hydrosilylation in water. An emulsion form of catalyst includes a dispersion in water of liquid catalyst, the main component of which is a platinum and alkenylsiloxane complex. The volumetric average particle size of liquid catalyst dispersed is less than or equal to 1 .mu.m. Hydrosilylation of an aliphatically unsaturated carbon-carbon bond by silicon-bonded hydrogen is carried out in water using the catalyst.