CIE colorimetry breaks down when lights produced by narrow band RGB-LEDs are matched with broad-band lights. A colour matching experiment was set up and matches in a number of parts of the chromaticity diagram have been made, to determine the magnitude of the discrepancy. Differences between visual and instrumental matches increase as one moves in the chromaticity diagram from yellowish white lights toward greenish and bluish lights. CIE TC 1-36 recently suggested newly defined cone fundamentals: Applying a transformation of these to a space similar to the CIE XYZ space enables a much better prediction of the matches to be made. The difference between the visual match and its instrumental prediction decreases by a factor of two or even more. The use of a cone fundamental based colorimetric system is recommended for LED colorimetry. (C) 2008 Wiley Periodicals. Inc.