How does the perception of a new phoneme contrast develop? In answering this question we consider two hypotheses: i) Acquired Distinctiveness: before learning, differences between and within phoneme categories are hardly discriminable. Through training, the phoneme boundary is learnt. ii) Acquired Similarity: before learning, differences between and within phoneme categories are well discriminated. Through training, only the phoneme boundary remains discriminable. In a pretest-training-posttest design, Dutch adults learnt the British- English pseudowords thif and sif: the first consonant in thif is not a phoneme of Dutch. Between pretest and posttest with materials from one speaker, participants were trained with speech from five other speakers. This forced listeners to form abstract phoneme categories. The results show that trained listeners performed better in the posttest than control listeners. However, in general the control group, who received no training, was difficult to distinguish from the trained listeners. With respect to the research question we found that discrimination levels increased as a result of training.