Abstract A new method to determine the group and phase velocity surfaces using only one image in phase contrast detected by phase sensitive scanning acoustic microscopy (PSAM) has been developed. Similar to earlier techniques, a focusing ultrasonic transducer is adjusted to a spot on a surface of a sample. A second similar transducer is scanned with the focus in the plane of the opposing planar surface. In the application presented here the group and phase velocity surfaces are derived from the observed phases of the transmitted ultrasonic waves. Numerical methods have been developed to achieve a continuous representation of the phases from the measured images. Subsequently the data is corrected for geometrical effects caused by the planar scanning sheet leading to a three-dimensional representation of the velocity surfaces. By additional procedures based on simple geometrical conditions, the phase velocities are derived in a straightforward way from the group velocities.