Application of the photon correlation technique to the measurement of sinusoidally perturbed fluid flows is described. An expression is derived for the shape of the count correlation function in terms of mean flow velocity and perturbation amplitude. Measurements have been made both in the wake behind a circular cylinder and with a mechanical flow simulator. These show that perturbation amplitudes can be measured by direct visual observation of the correlation displays. Maximum entropy transformation and curve fitting techniques are also described which produce results of higher accuracy.