The rate coefficients for the reactions of chlorine and fluorine atoms with acetonitrile have been measured using relative and direct methods. In the case of chlorine atoms the rate coefficient k(1) was measured between 274 and 345 K using competitive chlorination and at 296 K using laser flash photolysis with atomic resonance fluorescence. The rate coefficient measured at ambient temperature (296 +/- 2 K) is (1.15 +/- 0.20) x 10(-14) cm(3) molecule(-1) s(-1), independent of pressure between 5 and 700 Torr (uncertainties are 2 standard deviations throughout). This result is a factor of 6 higher than the currently accepted value. The results from the three independent determinations reported here yield the Arrhenius expression k(1) (1.6 +/- 0.2) x 10(-11) exp[-(2140 +/- 200)/T] cm(3) molecule(-1) s(-1). Product studies show that the reaction of Cl atoms with CH3CN proceeds predominantly, if not exclusively, by hydrogen abstraction at 296 K. The rate coefficient for the reaction of fluorine atoms with acetonitrile was measured using both the relative rate technique and pulse radiolysis with time-resolved ultraviolet absorption spectroscopy. The rate coefficient for the reaction of F atoms with CH3CN was found to be dependent on total pressure. The observed rate data could be fitted using the Tree expression with F-c = 0.6, k(0) = (2.9 +/- 2.1) x 10(-28) cm(6) molecule(-2) s(-1), and k(infinity) = (5.8 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), with a zero pressure intercept of (0.9 +/- 0.4) x 10(-11) cm(3) molecule(-1) s(-1). The kinetic data suggest that the reaction of F atoms with CH3CN proceeds via two channels : a pressure-independent H atom abstraction mechanism and a pressure-dependent addition mechanism. Consistent with this hypothesis, two products were observed using pulsed radiolysis with detection by UV absorption spectroscopy. As part of the product studies, relative rate techniques were used to measure k(Cl+CH2ClCN) = (2.8 +/- 0.4) x 10(-14) and k(F+CH2FCN) (3.6 +/- 0.2) x 10(-11) cm(3) molecule(-1) s(-1).