This article has a twofold purpose. First, to discuss the formation of the two types of complex words used to name instruments in Spanish (-(d)or nominals and V+N compounds), whose structures are proposed here within the framework of Distributed Morphology. Secondly, we will try to explain some of the difficulties in their acquisition, particularly those related to the selection of the corresponding verbal form of their bases. Stating the nature of these difficulties would help explain the acquisition of both forms and test the initial hypothesis of our research work: the greater structural complexity of V+N compounds determines the preference for derivation. The data used to investigate this issue has been collected through a series of structured interviews to three groups of children (ages 3 to 7) and form part of an experimental study. The analysis of the data reveals that the children use both forms to name instruments, showing a preference for derivation; they are conscious of the basic structures of these words and, in general, use them correctly. As regards the children's mistakes in the selection of the corresponding verbal forms, some are of a structural nature, some phonological and some phonological/encyctopaedic. No significant differences have been observed between the incorrect verbal forms selected for each type of word. Therefore, it can not be stated that one of the structures presents a greater degree of difficulty than the other; the preference for derivation would not be explained on structural grounds but on cognitive reasons instead.