This article presents research into derivational properties of onomatopoeias in English and in Slovak. Onomatopoeias are defined narrowly in our approach, being restricted to the direct imitation of sounds of extra-linguistic reality. Our sample of 40 onomatopoeic words consists of two sound types: the sounds of animals and the sounds resulting from various falls, strokes, and bursts. A derivational network was produced for each such word. The evaluation parameters comprise derivational capacity, maximum derivational network, saturation value, number of derivation orders, most productive semantic categories by order of derivation, typical combinations of semantic categories, and derivational processes. An evaluation of the networks enabled us to answer the question of whether onomatopoeias are productive word-formation bases, to compare the two Sound Types, and to compare onomatopoeia-based networks to those based on non-iconic vocabulary. These results contribute to a better understanding of the word-formation systems in the compared languages and of the status of onomatopoeias with regard to non-iconic vocabulary.