Magnetic tunnel junctions (MTJs) were fabricated using thin films of the half-metallic ferromagnet CrO2, employing semiconducting SnO2 tunnel barriers. Heteroepitaxial CrO2/SnO2 bilayers were grown on (100)-TiO2 substrates via chemical vapor deposition under atmospheric conditions. X-ray diffraction and transmission electron microscopy were used to confirm heteroepitaxy. A polycrystalline cobalt film forms the top magnetic electrode, yielding CrO2(001)/SnO2(001)/Co structures after patterning. Tunneling magnetoresistances (TMR) up to +14% at 5 K were observed. The sign of the TMR reverses for barrier thicknesses < 1nm, attributed to tunneling being dominated by Co-3d states at low thicknesses, and Co-4s states at larger thicknesses.