A case study is presented which describes the use of spectrographic measures to analyse the effectiveness of phonological intervention. Perceptual analysis showed that the child realized the phonemes /l/ and /j/ as the phone . However, acoustic analysis showed slight differences between his productions of /l/ and /j/. The child's development of differential phones involved systematic acoustic changes. Principles of coarticulation formed the basis of intervention which resulted in adult productions of /l/ and /j/. This study indicates the potential value of acoustic analyses for evaluating the efficacy of phonological intervention.