When gas condensate and oil come from wells into field gathering system it is difficult to obtain reliable data on the volume of stable condensate and oil because direct measurement of stable condensate volume is impossible. It is necessary to know exact volume of gas condensate so that severance tax could be applied to condensate, not to oil.To resolve the problem, unstable condensate volume change test was developed. Amount of unstable condensate is measured at wellhead by mobile test separator. The work included three stages: field tests; laboratory tests of gas and unstable condensate samples; systematization and generalization of materials obtained during first two stages. It was discovered that shrinkage factor is related to well parameters. The relationship was established empirically. For better correlation of field tests and laboratory test of different well samples, results were adjusted using correction system. Adjustments and statistical analysis of test results were embodied in the analytical form. Obtained materials help evaluate the difference of gas amount in unstable condensate calculated by formula, and actual gas amount discovered in samples from mobile test separator. Correlation was found between experimental data and test results of field laboratory. This will allow the use of mobile test separator to establish shrinkage volume factors for all producing wells.