The paper provides the results of an experimental investigation on the occurrence of the critical heat flux (CHF) under transient conditions, carried out at the Thermal Process Engineering Division (former Heat Transfer Laboratory) of ENEA Casaccia, ENE-MPE. Experiments were carried out using Freon 12 as a working fluid and a vertical, electrically heated test section 7.7 mm i.d. tube, 2.3 m long. Transient conditions refer to flow rate decrease, pressure decrease and heat flux increase. Variations of parameters were imposed separately (simple transients), keeping the other two parameters constant during the tests, or simultaneously, giving rise to complex transients as close as possible to the real situation. Data analysis has been achieved developing a computer code, ANATRA, able to simulate the behaviour of the CHF under transient conditions in vertical tubes, based on the local analysis method and on the quasi-steady approach. Experimental results and code predictions are detailed in the paper.