It is a long-held view that every syllable in Mandarin Chinese is a meaning-bearing unit, transcribed by a character. The syllables/morphemes can combine to form disyllabic compounds. I challenge this view by examining the UR learning of words that are purported to have undergone tone 3 sandhi: /T3 T3/ surfacing as [T2 T3]. I have developed a forced AABB reduplication diagnostic that can reveal what UR the speakers have learned. A survey of the judgement of six speakers has shown that certain disyllabic words listed as "T3 T3" in the dictionary are prone to being "underlearned" as /T2 T3/ by speakers. Specifically, compositionally opaque "T3 T3" words are the ones most likely to be "underlearned" as /T2 T3/. The phonological learning data suggest that many syllables in Mandarin disyllabic words are not meaning-bearing units, but merely part of a disyllabic morpheme.