The aim of the study was to investigate the feasibility of a blended learning approach to enhance students’ vocabulary knowledge and vocabulary knowledge retention in English as a foreign language (EFL) classrooms at the tertiary level in Thailand. This was to address challenges in relation to practice and use of English language, rote learning and memorisation, limited one-to-one interaction with peers and teacher, lack of learner-centredness, and low rate of knowledge retention. This study was conducted as a quasi-experimental design, employing the sample from four intact classes with a total of 146 students who registered in an English course at a university in Bangkok, Thailand. The sample was divided into experimental and control groups. The experimental group was exposed to the flipped classroom model, while the control group was taught in the traditional setting. Research tools consisted of an English language proficiency test, vocabulary pre-test, post-test, and delayed test, questionnaire, interviews and observations. The quantitative findings revealed a negative overall impact of the blended learning instruction on the experimental group, and on some particular classes and different academic majors. Gender differences and correlations occurred between language proficiency, vocabulary knowledge and knowledge retention. Qualitative results indicated that students and teachers had positive perceptions and attitudes towards feasibility of the approach. Although the blended learning method is perceived in a positive and feasible way, it may only be applicable for some particular groups or types of learners. Hence, different aspects regarding nature of learners and language learning should be taken into consideration, these include: language abilities, background knowledge, gender, academic majors, learners’ characteristics and capabilities, content and assessment, and selective types of technology
