Mixing by turbulent diffusion and secondary flow between the parallel subchannels of ducts simulating smooth, bare rod bundles has been investigated both experimentally and analytically. This paper, part 1, outlines the problem and reports experimental results; part 2 deals with the computer prediction of turbulent flow in rod bundles. The experimental work was performed on a long wind tunnel which was designed to achieve an axially unchanging, fully developed temperature profile at the tunnel exit. This is believed to be the first substantial investigation of inter-subchannel mixing using this technique. Detailed measurements were made over a range of Reynolds numbers and in three configurations simulating pitch to diameter ratios of 1.833, 1.375 and 1.1. The results of the experimental work confirm the major findings of previous investigations, in particular that the inter-subchannel mixing rates are considerably higher than predicted by simple diffusion theory, and are relatively insensitive to variations in the gap width between the rods. Effective diffusivities through the gap appear to be strongly anisotropic and there is no evidence of secondary flows.