The simplest Criegee Intermediate (CH2OO), a well-known biradical formed in alkene ozonolysis, is known to add across double bonds. Here we report direct experimental rate measurements of the simplest Criegee Intermediate reacting with C2-C4 alkenes obtained using the laser flash photolysis technique probing the recently measured B(1)A' <- X(1)A' transition in CH2OO. The measured activation energy (298-494 K) for CH2OO + alkenes is E-a approximate to 3500 +/- 1000 J mol(-1) for all alkyl substituted alkenes and E-a = 7000 +/- 900 J mol(-1) for ethene. The measured Arrhenius pre-exponential factors (A) vary between (2 +/- 1) X 10(-15) and (11 +/- 3) X 10(-15) cm(3) molecule(-1) s(-1). Quantum chemical calculations of the corresponding rate coefficients reproduce qualitative reactivity trends but overestimate the absolute rate coefficients. Despite the small E-a's, the CH2OO + alkene rate coefficients are almost 2 orders of magnitude smaller than those of similar reactions between CH2OO and carbonyl compounds. Using the rate constants measured here, we estimate that, under typical atmospheric conditions, reaction with alkenes does not represent a significant sink of CH2OO. In environments rich in C=C double bonds, however, such as ozone-exposed rubber or emission plumes, these reactions can play a significant role.