The positive and negative ion ultraviolet laser desorption time-of-flight mass spectra of [Ru3(CO)12] have been recorded, using a 337 nm N2 laser, over a range of experimental conditions. In the negative ion spectra the parent ion is not observed, but peaks corresponding to the trinuclear cluster ions [Ru3(CO)11]-, [Ru3(CO)10]-, and [Ru3(CO)9]-, derived directly from the parent by the successive loss of one to three CO groups, are present. In addition, peaks corresponding to cluster ions with nuclearities ranging from 2 to 11 are observed. The higher mass clusters in the negative ion spectra have considerably greater intensities than the trinuclear clusters, suggesting that stable higher nuclearity clusters have been generated during the laser desorption process. It would appear that, in general, these correspond to the series of carbonyl clusters derived from the thermal decomposition of [Os3(CO)12] that have yet to be isolated for ruthenium. In the positive ion spectra peaks corresponding to [Ru3(CO)13]+, [Ru3(CO)12]+, [Ru3(CO)11]+, and [Ru3(CO)10]+ are observed. Other peaks corresponding to species containing from four to six ruthenium atoms are also present. In addition, the appearance of peaks corresponding to the dinuclear species [Ru2(CO)8]+ and [Ru2(CO)9]+ indicates that some fragmentation takes place. Representative mass spectra are shown and some mechanistic proposals are given concerning the possible ionization mechanisms and the effect these have on the clustering processes. Related phenomena have been observed in similar experiments on [Fe3(CO)12] and [Os3(CO)12], and their spectra are compared with those of [Ru3(CO)12].