Experimental investigations on critical heat Aux have been performed in circular tubes of different diameters ranging from 2 to 16 mm. The model fluid Freon-12 was used as working fluid due to its low latent heat, low critical pressure and well-known properties. About 1700 data points have been obtained in a large range of parameters : pressure 1.0-3.0 MPa, mass flux 1.0-6.0 Mg m(-2) s(-1) and exit steam quality -0.75 to +0.59. It was found that the present rest results coincide well with the data sets available in the literature. A very good agreement between the test data in the 8 mm diameter tube and the CHF look-up table shows that the test results in Freon-12 can be well transferred to water conditions by using the Ahmad fluid-to-fluid scaling law. The experimental data reveal that the effect of the tube diameter on CHF cannot be described by the equations or models available in the literature with sufficient accuracy.