The invention relates to a method of monitoring a start-up sequence of a turbomachine that comprises a compressor equipped with a rotor, a starter capable of driving the rotor in rotation and a combustion chamber. The start-up sequence comprises a first phase during which the starter increases the rotation speed of the rotor up to an instant at which fuel is injected into the turbomachine combustion chamber, and a second phase after the first phase that terminates when the starter stops driving the rotor. The method includes: acquisition (ACQ) of a signal representative of the rotation speed of the rotor during the start sequence; detection (DRP1, DRP2, INT) of an instant at which there is a sudden change in the variation of said signal with time, the sudden change instant thus detected being deemed to be the instant at which an air-fuel mix is ignited in the combustion chamber.