Modern ink-jet printers often provide more than 4 print heads. The additional channels can be used for faster printing, diluted process colours or additional dyes (green, orange). A proprietary gamut evaluation program is used to investigate the benefits of diluted inks vs. the addition of colours. The experiments are made using a four- ink printer. The eight-ink system has to be simulated by specially designed colour wedges that are printed with 4 inks each and are later assembled into colour space ribs. The colour gamut is calculated as the colour space volume sustained by the ribs using a spectral interpolation technique. The evaluation includes colour gamut size, tone reproduction and image quality criteria as well as permanence considerations.