Abstract: Single-step, pressureless process for preparing dimethylhydrogenchlorosilane (DMS.dbd.(CH.sub.3).sub.2 HSiCl) with a content of at least 97% by weight, the product being free of unsaturated hydrocarbons, by adding hexachloroplatinic acid in a catalytic amount to the first-run mixture of the direct synthesis of methylchlorosilanes, distilling between 33 and 37.degree. C., the addition of the acid taking place before fractionation is carried out. The so obtained DMS is used for the modification of silicone products, particularly to improve specific parameters, as for instance, resistance to reversion, oil resistance and flame resistance.