This study investigates the nature of phonological impairment in 20 pre-and earlyschool Portuguese children with phonologically based Speech Sound Disorders (SSD). The phonological profile of these children was compared with the performance of 232 younger typically developing (TD) controls. Results showed differences in phonological acquisition of children with phonologically based SSD, namely a low percentage of correct consonants score and a higher percentage of occurrences of typical phonological processes. Children with phonologically based SSD also used phonological processes (e.g., backing and initial consonant deletion) that are considered unusual in normally developing children and were not observed in the TD group. Results for SSD children also showed that early acquisition consonants were substituted by later acquisitions. Children with SSD presented differences in phonological acquisition, unusual phonological processes, and replaced early acquisition consonants by later acquisition consonants. This knowledge is useful for speech and language therapists in the diagnosis and treatment of SSD.