Abstract ORGANOPOLYSILOXANES USEFUL AS HYDRAULIC FLUIDS COMPRISE ORGANOPOLYSILOXANES IN WHICH A MAJORITY OF THE SILOXANE GROUPS CONTAIN METHYL RADICALS AND HIGHER ALKYL RADICALS, AND A MINOR PROPORTION OF THE SILOXANE GROUPS CONTAIN SILICON-BONDED TERTIARY BUTYL-SUBSTITUTED HYDROXYARYL RADICALS ATTACHED TO SILICON THROUGH A DIVALENT PROPYLENE RADICAL. OPTIONALLY, A MINOR AMOUNT OF ARYL AND ARALKYL RADICALS CAN ALSO BE PRESENT ON THE SILOXANE GROUPS. THESE COMPOUNDS ARE PREPARED BY REACTING POLYSILOXANES CONTAINING METHYLHYDROGEN UNITS AND PHENYLHYDROGEN UNITS WITH HIGHER ALKENES AND HYDROXYARYL COMPOUNDS CONTAINING BOTH NUCLEAR SUBSTITUTED ALLYL RADICALS AND NUCLEAR SUBSTITUTED TERTIARY BUTYL RADICALS.