Aerosol water mass is important in resolving aerosol composition, assessing aerosol light-scattering effect, and eventually in evaluating solar radiation received by the earth. This work presents a novel method to detect aerosol water mass for changing relative humidity (RH) at a stable room temperature. The target aerosols are generated by atomizing the 1.5% (wt/wt)(NH4)2SO4 or Na2SO4 solution and collected on a Teflon filter. Aerosol particles are then exposed to the moistened air and, later, purged by a pure helium flow. Next, the purged water vapor is then directed through a thermal conductivity detector installed in an elemental analyzer for detection. Aerosol water is calculated from the difference between the system water associated with the blank filter and that with the particle deposited filter. System water is subsequently checked and is attributed to the retaining of water vapor by valves, sampling lines, blank filter, filter back-up screen, and the empty filter holder in the sampling system. As the results show, (NH4)2SO4 particles deliquesce at around 80% RH to contain 8 mol H2O/mol solute and Na2SO4 particles become saturated solution droplets containing about 13 mol H2O/mol solute at around 85% RH. The instrument detection limit for water released in the system is 24 μg and the quantitative detection limit for (NH4)2SO4 particles is around 0.77 μmol. This method provides a practical approach to assess the water mass associated with the aerosol collected in the field.