Timbre spaces represent the organization of perceptual distances, as measured with dissimilarity ratings, among tones equated for pitch, loudness, and perceived duration. A number of potential acoustic correlates of timbre-space dimensions have been proposed in the psychoacoustic literature, including attack time, spectral centroid, spectral flux, and spectrum fine structure. The experiments reported here were designed as direct tests of the perceptual relevance of these acoustical parameters for timbre dissimilarity judgments. Listeners presented with carefully controlled synthetic tones use attack time, spectral centroid, and spectrum fine structure in dissimilarity rating experiments. These parameters thus appear as major determinants of timbre. However, spectral flux appears as a less salient timbre parameter, its salience depending on the number of other dimensions varying concurrently in the stimulus set. Dissimilarity ratings were analyzed with two different multidimensional scaling models (CLASCAL and CONSCAL), the latter providing psychophysical functions constrained by the physical parameters. Their complementarity is discussed.