In this paper we present a reversible watermarking technique for digital images. In our context, reversibility refers to the ability to restore the original image by the watermark detector. The technique is based on a transformation function that introduces 'gaps' in the image histogram. The 'gaps' are used to encode the watermark. Several experiments revealed that a relatively high embedding rate may be achieved. For a set of test images we obtained embedding rates between 0.06 and 0.6 bits per pixel at PSNR levels of 45-50.