H 2MoO 4 and H 2WO 4 upon direct laser desorption produce small clusters in the gas phase, extending up to 4500 and 7200 Da, respectively. The clustering patterns are quite similar for the two systems. The mass difference between successive peaks differ with increasing mass number as different kinds of structures become possible. A distinct planar to three-dimensional structural cross over is observed around the M 8 cluster. Possible structures of these clusters are presented. MoO 3 and WO 3 were also used as starting materials, which yielded the same clusters. © 2004 Elsevier B.V. All rights reserved.