An acoustic method of measuring critical temperatures and pressures of thermally unstable substances is described. In the cell with the liquid under investigation there is a wire probe, 0.02 mm in diameter, which is heated by electric-current pulses with a duration of 0.025 to 0.25 ms. At the moment of liquid boiling-up in the layer that surrounds the probe, one can observe an acoustic wave, which is registered by a ceramic piezoelectric element on a base (PbZrO3 + PbTiO3). The probe temperature at the moment of boiling-up is determined from its resistance. With an increase of the pressure in the cell with the substance under investigation the amplitude of an acoustic signal decreases. The cessation of boiling-up and disappearance of the acoustic signal show that the critical state is reached. A more precise critical pressure is found by linear extrapolation of the amplitude of the acoustic signal to zero in an amplitude-log (pressure) plot. The paper reports the experimental critical temperatures T-c and the critical pressures p(c) of reference substances: n-hexane, n-heptane, n-decane, benzene, and of thermally unstable n-alkanes: docosane and tetracosane, and also the critical curve of the system n-hexadecane-benzene. The critical constants of reference substances were measured by this acoustic method with an error that does not exceed 0.015 T-c and 0.015 p(c).