The reaction of atomic boron, B(P-2), with the simplest alkene, C2H4, has been investigated under single collision conditions in crossed beam experiments with mass spectrometric detection. Our experimental data clearly showed that the atomic boron versus hydrogen exchange reaction led to molecule(s) of gross formula C2H3B via bound intermediate(s). According to the experimentally derived fraction of the available energy released as product translational energy, we propose that an important reaction pathways is the one leading to the borirene plus atomic hydrogen and/or the one leading to ethynylborane plus atomic hydrogen. The experimental results are accompanied by electronic structure calculations of the relevant potential energy surface and RRKM estimates of the product branching ratio. According to RRKM calculations, within the limit of complete energy randomization, the three isomers borirene, BH = C = CH2 and BH2-C CH, are all formed, with BH2-C CH being the dominant one. The discrepancies between the trend of the product translational energy distributions and the picture emerging from RRKM estimates are a symptom that a statistical treatment is not warranted for this system.