A quick and easy method for monitoring phase separation in glasses based on transmittance spectra is described. The spectra are recorded after different intervals during heat treatment of doped or undoped glass samples and chromaticity x-and y-values are obtained from them. The results can be depicted graphically in x/y chromaticity diagrams for various doping levels and/or exposure times. The influence of dopant ions on the phase separation becomes readily evident when the results for doped and undoped samples of the same host glass composition are compared. The glass samples presented are ternary sodium borosilicate glasses; the effect of doping with Nd³⁺ and P⁵⁺ is discussed.