Abstract. Wavelet packet decompositions generalize the classical pyramidal wavelet structure. We use the vast amount of possible wavelet packet decomposition structures to create a secret wavelet domain and discuss how this idea can be used to improve the security of watermarking schemes. Two methods to create random wavelet packet trees are discussed and analyzed. The security against unauthorized detection is investigated. Using JPEG and JPEG2000 compression we assess the normalized correlation and Peak Signal to Noise Ratio (PSNR) behavior of the watermarks. We conclude that the proposed systems show improved robustness against compression and provide around 21046 possible keys. The security against unauthorized detection is greatly improved.