The design of tight-lattice pressurized water reactors requires the knowledge of CHF in tight rod bundles. Experimental investigations on CHF behavior in tight hexagonal 37-rod bundles were performed by using the model fluid Freon-12. About 400 CHF data points have been obtained in a range of parameters: pressure 1.0-2.7 MPa, mass flux 1.4-4.5 Mg/m(2) s and bundle exit steam quality -0.4 to +0.2. It is found that the effect of different parameters on CHF in the tight rod bundle is similar to that in tube geometries. The present test results agree also quantitatively well with the CHF data obtained in tubes of comparable hydraulic diameters. Some in the open literature well known CHF correlations proposed for rod bundles under-predict the test results significantly. (c) 2005 Elsevier Ltd. All rights reserved.