This paper examines whether students involved in CLIL are able to learn content through the medium of L2 and simultaneously exhibit significant gains in L2 vocabulary knowledge. Two experiments were set up in two public primary schools. Two groups of 6th grade students participated in each experiment. The first group was taught three 80-minute Science lessons through the medium of L2 English, while the second group was taught the same content through the medium of L1 Greek. The outcomes demonstrated a significant effect of CLIL (p= 0.001) on L2 vocabulary knowledge of the experimental groups, which outperformed the control groups that were not exposed to CLIL. A significant effect (p= 0.000) of treatment on content knowledge was shown for both experimental and control groups. Observation of three video-taped Science lessons provided more information about the learning processes allowing benefits for CLIL students. Avenues for further related research are discussed. (Contains 1 figure.)