Diamond was co-doped with sulfur and boron during deposition using hydrogen sulfide and trimethylboron in the source gases. Secondary ion mass spectroscopy (SIMS) confirmed the presence of sulfur and showed that the sulfur incorporation is concentrated in the near-surface region. Also, examination by scanning tunneling spectroscopy (STS) of a cleaved cross-section showed greater conductivity in the near-surface region. Hall effect, STS, the thermoelectric effect and Mott-Schottky analysis confirmed the n-type conductivity for films grown with sulfur and limited amounts of boron. The source of the donors is not known and could arise from defects or impurity bands. Additionally, the properties of a pn-junction using co-doped diamond for the n-type layers are discussed.