The invention relates to a method for measuring a flow velocity (v) of a gas stream (14) featuring the steps: (a) time-resolved measurement of an IR radiation parameter (E) of IR radiation of the gas stream (14) at a first measurement point (P1) outside of the gas stream (14), thereby obtaining a first IR radiation parameter curve (Eg1,1(t)), (b)time-resolved measurement of an IR radiation parameter (E) at a second measurement point (P2) outside of the gas stream (14), thereby obtaining a second IR radiation parameter curve (Eg1,2(t)), (c) calculation of a transit time (τ1) from the first IR radiation parameter curve (Eg1,1(t)) and the second IR radiation parameter curve (Eg1,2(t)), in particular by means of cross-correlation, and (d) calculation of the flow velocity (vG) from the transit time (τ1), (e) wherein the IR radiation parameter (Eg1) is measured photoelectrically at a wavelength (g1) of at least 780 nm, and (f) a measurement frequency (f) is at least 1 kilohertz.