Polymethylhydrosiloxane (PMHS) is a polymer with the general structure [−CH₃(H)Si−O−]. It is used in organic chemistry as a mild and stable reducing agent easily transferring hydrides to metal centers and a number of other reducible functional groups. A variety of related materials are available under the following CAS registry numbers 9004-73-3, 16066-09-4, 63148-57-2, 178873-19-3. These include the tetramer ((MeSiHO)₄), copolymers of dimethylsiloxane and methylhydrosiloxane, and trimethylsilyl terminated materials.

This material is prepared by the hydrolysis of monomethyldichlorosilane CAS#: 75-54-7:

The related polymer polydimethylsiloxane (PDMS) is made similarly, but lacking Si−H bonds, it exhibits no reducing properties. Dimethyldichlorosilane CAS#: 75-78-5 is then used instead of monomethyldichlorosilane CAS#: 75-54-7.

Illustrative of its use, PMHS is used for in situ conversion of tributyltin oxide to tributyltin hydride: