Perception of instrumental blend is important for understanding aspects of orchestration, but no work has studied blends of impulsive and sustained instruments. The first experiment identified the factors that influence the rating of blendedness of dyads formed of one sustained sound and one impulsive sound. Longer attack times and lower spectral centroids increased blend. The contribution of the impulsive sound’s properties to the degree of blend was greater than that of the sustained sound. The second experiment determined the factors that influence similarity ratings among dyads. The mean spectral envelope and the attack time of the dyad best explained the dissimilarity ratings. However, contrary to the first experiment, the spectral envelope of the sustained sound was more important than that of the impulsive sound. Multidimensional scaling of dissimilarity ratings on blended dyads yielded one dimension correlated with the attack time of the dyad and another dimension whose spectral correlate was different for two different clusters within the space, spectral spread for one and spectral flatness for the other, suggesting a combined categorical-analogical organization of the second dimension.