This article examines the effectiveness of watching captioned authentic videos for incidental vocabulary learning by comparing it to intentional learning of new lexis through a set of tasks and a control group. For this purpose, 32 Spanish learners of English were distributed among three groups: intentional, incidental and control. The intentional group took part in a one-hour classroom teaching session, whereas the participants from the incidental group were exposed to 5 hours of captioned authentic video. The control group was used as a reference. The performance of the three groups was compared in three vocabulary tests, which were developed to measure the learning of three different aspects of word knowledge: form recognition, meaning recognition and written use in a sentence. While no significant difference was observed between the performance of the incidental group and the control one, the intentional group demonstrated considerably higher scores in the three vocabulary tests. The effectiveness of incidental vocabulary learning through watching 5 hours of captioned authentic videos is discussed in terms of the variables that could have affected the learning process.