The upper critical field H c 2 (T) of sintered pellets of the recently discovered MgB 2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between T c 39K and T=3K 0.1T c . The temperature dependence of the upper critical field, H c 2 (T), shows a positive curvature near T c similar to that found for the borocarbides YNi 2 B 2 C and LuNi 2 B 2 C indicating that MgB 2 is in the clean limit. The irreversibility line was consistently determined from dc magnetization measurements and from the imaginary component of ac susceptibility. The irreversibility field was found to increase up to 8.5 T at 10 K.