The influence of spacer grids on the heat transfer in gas-cooled rod bundles was determined experimentally for the first time over a wide range of parameters. The experimental investigations were carried out with a smooth and a rough rod bundle for Reynolds numbers between 600 and 2 × 105. The measured range of Reynolds numbers covered the transition from laminar to turbulent, the transition from hydraulically smooth to rough, and fully rough flows. In gas cooling, artificial roughnesses on the rod surfaces are used to disturb the viscous sublayer, which acts as an insulator because of the low thermal conductivity of gases. For this investigation, a two-dimensional rectangular roughness was used, which had an optimum heat transfer characteristic. The blockage factor ∊ was varied between 25 and 35%. These values are typical of flow blockages due to spacer grids in gas-cooled fast reactors. The measurements were carried out from 10 Dh upstream to 33 Dh downstream of the spacer grid. The measured range covered the zone of heat transfer influenced by the spacer grid. The measurements showed heat transfer to be improved by spacer grids in all cases investigated. On the basis of the measurements, empirical correlations could be established for the influence of the spacer grid on heat transfer in terms of the measured parameters, i.e., Reynolds number, blockage factor, and the type of heat transfer surface. These empirical correlations can be directly used in computer codes for analysis of the thermodynamics and fluid dynamics of gas-cooled rod bundles.