The nonlinear coefficients of the amplitude equations for the stationary, o
scillatory and codimension-2 point bifurcations are calculated for isotherm
al double diffusive convection with cross-diffusion. The locations of the t
ricritical point for the stationary instability and the codimension-2 point
are also found. Thereby the separation between these points in parameter s
pace can be calculated as a function of rescaled cross-diffusion constants.