The effects of an inappropriately chosen speed-of-sound in photoacoustic imaging reconstructions are to cause blurring of images and impairment of contrast. Here we outline a new methodology to measure the speed-of-sound in a photoacoustic imager with little or no additional cost and without the need to perform extra measurements. The method uses a strong absorber of light which is placed in the path of the light illuminating the sample. This acts as a source of ultrasound whose interaction with the sample can be measured at the far-end of the sample using the same ultrasound detector used for photoacoustics. This yields time-of-arrival measurements of the ultrasound transient at the multi-element detector. Such measurements are made at various angles around the sample in a computerized tomography approach. Reconstruction of the time-of-arrival or speed-of-sound tomogram of the object can be made by implementing a fan-beam projection reconstruction algorithm. We present the concept and validate the method on a speed-of-sound phantom.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.