A study of speaker intelligibility involving 45 speakers from a homogeneous accent group was carried out with adult, 7-8 year old and 11-12 year old listeners. Ranking of speakers according to their intelligibility was remarkably consistent across listener groups, thus demonstrating that inherent speaker characteristics are the primary contributor to intelligibility. In a second study, listener ratings of speakers with highest and lowest intelligibility on a number of dimensions suggested that spectral differences and differences in the quality of vocal fold excitation may be correlated with intelligibility. Physical measures of long-term spectrum and fundamental frequency were carried out. These did not reveal consistent predictors of intelligibility, but relationships were observed between these and subjective judgements of voice characteristics.