The effect of surface tension of a deformably free upper surface on the onset of convective instability in a doubly diffusive fluid layer is investigated with the linear stability theory. The eigenvalue problem is numerically solved, employing the Runge-Kutta-Gill’s method of order four and making use of the Broyden’s method for correction of the starting values during the integration process. The results show that the effect of Crispation number, C, becomes very sensitive on determining the possible mode of convective instability. The surface tensile and thermosolutal modes can coexist stationarily at C = 8.44 x 10(-4). For C < 8.16 x 10(-4), the whole system is dominated by the oscillatory mode. One of the interfacial effects, associated with the Bond number, Bo, do increase critical conditions of both stationary and oscillatory modes. The Marangoni number, M, and the solutal Marangoni number, M(s), play important roles on causing the onset of the convective instability and, in turn, reinforce each other mutually.