The purpose of this mixed-methods study was to examine the impact of Web multimodality plus dialogical interactions in the acquisition and retention of novel lexical items among EFL students under a social constructionist framework. The lexical acquisition of 107 1st-year English majors at the University of Costa Rica was analyzed through Simultaneous Multiple Linear Regression and discourse analysis. Treatment A group, exposed to multiple Web input sources and allowed to discuss their findings dialogically, was compared to an only-Web group and a Control group. The difference in means between pre and posttests indicates that scores increased after each treatment. The Control group showed a decrease between pre and posttest mean scores. The results of the regression were statistically significant and the marginal mean of the Web plus dialogue group was statistically different from the means of the Web-only and Control groups (p < 0.05). Other variables such as learning preference, language use and background did not have any predictive power in the model. The qualitative section showed that students positively appraised the use of Dictionary.com and Google Translate in the search for meaning. This finding reinforces the validity of using the Web to present novel vocabulary to EFL students. The examination of 64 learners' oral interactions demonstrated that the majority of their interventions indicated co-construction of knowledge supporting Gunawardena et al.'s model (1997, 1998) and the use of Repetition, Code mixing, and Social content strategies. The progression of learners' interactions along the different phases of the co-construction model provided evidence of meaning creation and accounted for the development of a semantic framework for the comprehension of the target vocabulary through the collaboration of the different participants. The results of this study have pedagogical implications by informing practitioners about student preferences when integrating multimodalities into FL instruction and the intrinsic value of dialogical interactions in the social construction of meaning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]