An improved procedure for controlling the acidity of phosphate ester fluids is described. Water in phosphate ester fluids causes a hydrolysis reaction to occur which increases the acidity of the fluid and eventually leads to fluid degradation. Currently, this problem is addressed through the use of acid adsorbing and/or neutralizing agents such as Fuller’s earth or activated alumina. These agents are designed to neutralize the acidic species formed in the hydrolysis reaction and not to remove water which is the cause of the problem. In contrast, the procedure described herein relies on water removal to prevent hydrolysis of the fluid. Removal of water is accomplished with a spinning disc purifier, which employs the vacuum dehydration/mass transfer principle. In addition, the purifier is equipped with a 3 µm (3> 2000 rated) filter for particulate removal. Results of the studies at East and West coast power plants operated by two major utilities indicate that by maintaining low water and particulate content in the fluid, the purifier effectively reduced the rate of phosphate ester hydrolysis. These data therefore suggest that the service life of both the fluid and the neutralizing agent can be extended beyond the present two-year period observed on average in the power generation industry.