Theoretical analysis of the development of surface convection in the transient process of gas absorption by an initially motionless layer of liquid was carried out. The main resistance to mass transfer is concentrated in the liquid phase. Convective instability is caused by the temperature dependence of the surface tension. Linear analysis showed a strong dependence of the critical time on the viscosity of the liquid. On the basis of the simplified nonlinear model that does not use semi-empirical assumptions, approximate estimates of the critical time from physical characteristics are obtained. The problem was solved in a two-dimensional formulation. The concentration of the absorbed substance is represented as the sum of the three terms of the Fourier series on the coordinate directed along the gas - liquid interface. Equations for the temperature and the fluid velocity are linearized. Nonlinearity was taken into account only in the equations for concentration. The calculations are compared with known experimental data.