Partial pressure of ammonia was calculated for a number of points in the phase diagram H2SO4 - H2O-NH3. It was found that the pairs of values PNH3 and relative humidity determine if the system will be present as solid phases or as solution. In addition, the partial pressures for HNO3 and HCl werer calculated in teh system H2SO4 - H2--NH3-HNO3-HCL in which the concentration of HNO3 and CL- was small in relation to H2SO4. These calculations showed that a nitrate concentration amounting to 0.1 of the total sulphate concentrations could be present in the system only if the acid was neutralised to 90-95 %. The results can be utilised to predict, by means of measuremts of gaseous components in air, the equilibrium composition of a sulphuric acid aerosol