A method to quantify gamut based on equiluminance planes is applied to characterize ink jet media and ink sets. Due to a mixture of spectral interpolation and interpolation in L*,a*,b* the base set of experimental data can be much smaller if only L*,a*,b* interpolation is used. The robustness of the interpolation to the reduction of data points in hue direction is demonstrated. The method is applied to the characterization of different ink jet ink sets on photo glossy media. The effect of media gloss, paper color, printing pattern and colorants is shown. The influence of the spectral measurement geometry on gamut has to be taken into account when colors are compared on samples with different surface finish. Printed colors may change over time due to the diffusion of dyes inside the layer. A test pattern was developed to quantify dye diffusion of ink jet prints under varying environmental conditions.