This paper reports on a study that was carried out to investigate the effect of pair work on a word-building task in two EFL classes. Forty Iranian adult students participated in this study. The participants in the experimental group completed the word-building task in pairs following the Think-Pair-Share technique, whereas the participants in the control group did the same task individually. Results of the data analysis showed that the participants in the experimental group achieved significantly higher scores on the given task than the participants in the control group. This indicates that the students' joint efforts while collaborating with each other are likely to result in co-construction of morphological knowledge.