Measurements have been made to determine the long-lived component,τ 2, and its intensity,I 2, of positrons decaying in normal and degassed silicone fluids of different viscosity grades. The results show that the lifetime,τ 2, of the triplet positronium decreases as the length of the chain of the silicone fluids increases, though the change is quite small. No dependence of the intensity,I 2, of the triplet positronium on molecular structure has been observed. No effect of the dissolved oxygen on triplet-positronium intensity has been observed. The large variation inτ 2 obtained by Grayet al. for normal aliphatic hydrocarbons having different chain lengths can be interpreted as arising mostly from density changes.