An experimental study is conducted to measure the transient characteristics of the local boiling process close to the heated wall. Nucleate and film boiling two-phase parameters were measured for power oscillations in a small horizontal heater immersed in stagnant liquid. A method for measuring fast temporal variations of void fraction and the interfacial impact rate is presented. Ensemble averages of the data have been carried out to obtain time evolution of the void fraction and the interfacial impact rate at the probe tip. R-113, R-11 and water have been used as working fluids, finding important differences between the behavior of the freons and water at the boiling crises. The critical heat flux in freons is associated with a sudden transition in the impact rate and the void fraction, which supports the theory of hydrodynamic instability. On the other hand, the experiments in water suggest the existence of two transitions : a hydrodynamic transition from bubbling regime to "mushroom" regime followed by a liquid film dryout type of CHF, in agreement with the theory of Haramura & Katto.