It is found that the mass loss rate derived from S 2 5 /S 1 2 is generally greater than that from the OH flux. This suggests an increasing loss rate with time. It is found that the gas-to-dust ratio decreases during the evolution Of OH/IR stars. An empirical relation between the gas-to-dust ratio and the dust mass loss rate is derived, leading to a formula for calculating the total mass loss rate directly from the IRAS 60 μm flux. OH/IR stars with silicate emission have a larger range of mass loss ratio, from 10 - 7 to 10 - 5 M /yr; those with silicate absorption, a smaller range between 10 - 5 and 10 - 4 M /yr. So a large increase in the mass loss rate takes place during the emission phase.