Background: The effectiveness of two treatment approaches (phonological therapy and articulation therapy) for treatment of 14 children, aged 4;0-6;7 years, with phonologically based speech-sound disorder (SSD) has been previously analysed with severity outcome measures (percentage of consonants correct score, percentage occurrence of phonological processes and phonetic inventory). Considering that the ultimate goal of intervention for children with phonologically based SSD is to improve intelligibility, it is curious that intervention studies focusing on children's phonology do not routinely use intelligibility as an outcome measure. It is therefore important that the impact of interventions on speech intelligibility is explored. Aims: This paper investigates the effectiveness of the two treatment approaches (phonological therapy and articulation therapy) using intelligibility measures, both in single words and in continuous speech, as the primary outcome. Methods and Procedures: Fourteen children with phonologically based SSD participated in the intervention. The children were randomly assigned to phonological therapy or articulation therapy (seven children in each group). Two assessment methods were used for measuring intelligibility: a word identification task (for single words) and a rating scale (for continuous speech). Twenty-one unfamiliar adults listened and judged the children's intelligibility. Reliability analyses showed overall high agreement between listeners across both methods. Outcomes and Results: Significant improvements were noted in intelligibility in both single words (paired t(6) = 4.409, p = 0.005) and continuous speech (asymptotic Z = 2.371, p = 0.018) for the group receiving phonology therapy pre-to post-treatment, but no differences in intelligibility were found for those receiving the articulation therapy pre-to post-treatment, either for single words (paired t(6) = 1.763, p = 0.128) or continuous speech (asymptotic Z = 1.442, p = 0.149). Conclusions and Implications: Intelligibility measures were sensitive enough to show changes in the phonological therapy group but not in the articulation therapy group. These findings emphasize the importance of using intelligibility as an outcome measure to complement the results obtained with other severity measures when exploring the effectiveness of speech interventions. This study presents new evidence for the effectiveness of phonological therapy in improving intelligibility with children with SSD.