Abstract Experimental and theoretical investigations have been performed on critical heat flux ( CHF ) and turbulent mixing in tight, hexagonal, 7-rod bundles. Freon-12 was used as working fluid due to its low latent heat, low critical pressure and well known properties. It has been found that the two-phase mixing coefficient depends mainly on mass flux. It increases with decreasing mass flux and ranges from 0.01 to 0.04 for the test conditions considered. More than 900 CHF data points have been obtained in a large range of parameters: pressure 1.0–3.0 MPa and mass flux 1.0–6.0 Mg/m 2 s. The effect of different parameters on CHF has been analysed. It has been found that the effect of pressure, mass flux and vapour quality on CHF is similar to that observed in circular tubes. Nevertheless, the CHF in the tight rod bundle is much lower than that in a circular tube of the same equivalent hydraulic diameters. The effect of wire wraps on CHF is mainly dependent on local vapour qualities and subsequently on flow regimes. Based on subchannel flow conditions, the effect of radial power distribution on CHF is small. Comparison of the test results with CHF prediction methods underlines the need for further work.