Content and Language Integrated Learning (CLIL) has become the focus of Foreign Language (FL) research within the last decades. CLIL provides a more complete, naturalistic, and meaningful context to FL learning, which has proven to bring many benefits to learners, such as a higher motivation and promotion of creativity, and better results in receptive skills, vocabulary, morphology and fluency. Nevertheless, most CLIL research has focused on primary and secondary level students and, thus, more research is needed with younger learners, namely, pre-primary students. The present study examines the learning of FL vocabulary in pre-primary learners following a soft-CLIL program, as compared to their same age peers following Formal Instruction (FI) of English. Over the course of six months, pre-primary students of two grades, namely 4- and 5-year-old students (N=155), took part in such program, aiming at teaching two curricular preschool units, traditionally taught in the mother tongue (L1), in English in the FL sessions. A longitudinal study was conducted, and students were administered a general vocabulary level pre-test, as well as a target words receptive vocabulary post-test after the two units had been worked on. The focus of the research was on receptive vocabulary acquisition, but age and word frequency effects were also analyzed. Results showed positive tendencies in receptive vocabulary development through soft-CLIL, although not statistically significant. A significant frequency effect was found, indicating that high-frequency words are recalled more easily than lower-frequency ones, but no significant differences were found when comparing learners from the two grades.