Mass transfer in the absorption of lean NH3 with water spray is examined under various experimental condition, using a small spray column. The absorption rates of NH3 are affected by both gas- and liquid-phase mass transfer resistances in the vicinity of the nozzle exit. The observed gas phase diffusion fluxes of NH3 show good agreement with the empirical correlation by Rant-Marshall, Taking the effect of gas- and liquid phase mass transfer resistances on the absorption rates into consideration, a new simulation technique is proposed for the absorption of lean gas using a small spray column, The present model can accurately predict the amount of absorbed NH3 by water spray.