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Abstract The purpose of the longitudinal study reported here was to determine the developmental relation between the two systems of expression available to the young child in the period of early language learning: affect and speech. Two achievements in language were identified for a group of 12 infants: First Words, at the beginning of the single-word period (mean age about 13 months), and a Vocabulary Spurt, which occurred toward the end of the period (mean age about 19 months). Affect expression was coded continuously in the stream of the infants' activity as they and their mothers played with groups of toys and ate a snack. The occurrence of words was examined in relation to the expression of affect and the results of this study concern developments in the integration of these two forms expression. The children's words occurred closely in time with their expression of emotionally toned affect. We concluded, then, that they were learning words to express what their feelings were about even though none of the actual words they said were emotion terms. However, the cognitive requirements for emotional expression and expression through speech resulted in several constraints on their integration. Words were said with neutral affect expression most often, with a peak in emotional expression in the moments immediately after words but a decrease in emotional expression before words. The peak in emotional expression with words was significantly greater, and the pre-word dip in emotional expression was significantly less, at the Vocabulary Spurt than at First Words. Thus, the two systems of expression converged in the period of single-word development as the children came to be able to say words with emotional affect. However, the words that were said together with emotional expression were said with positive rather than negative valence, with low rather than heightened intensity, and were among the most frequent and earliest learned words. These results are discussed in terms of the acquisition of language for expression and the different cognitive requirements for expression through affect and speech.