This paper focuses on examining the use of the Duolingo App as a supplement to the traditional ESP courses in Secretarial Studies, for Advanced ESP learners Classes of Lycée Technique et Professional de Porto Novo (LTP-PN), in the Benin Republic. The Objective is to assess substantial improvement of Learners Oral Communication skills after 3 months of studious use of mobile language learning apps. An experimental mixed-method research design in two stages have been used. During the first stage, quantitative data was collected using questionnaires administered to Ten (10) ESP teachers and Forty (40) randomly selected ESP secretarial studies learners of LTP-PN. Qualitative data was also gathered during classroom observations. In the second stage, the experimentation process involving the Experimental Groups (EG), which received the treatment, that is "the use of Duolingo for 15 Mn", and the Control Group (CG) were tested. The data was collected and computed using one way ANOVA test in SPSS 26. The result shows a statistical significance between the use of Duolingo and Secretarial Studies ESP learners' oral proficiency improvement. This paper further suggested that mobile learning apps such as Duolingo should be used as a supplement to the traditional ESP courses. It also pointed out the necessity for ESP teachers to invest in their professional development rather than waiting for the government.