Hot pressing of TiB2 has been investigated with particular emphasis on the evolution of secondary phases originating from the initial surface oxide layer on the TiB2 powders. Carbothermal reduction of the surface oxides during sintering was also investigated by adding carbon to the TiB2 powder. TiO1-xCx was shown to be the main secondary phase in hot-pressed TiB2, and carbon was shown to strongly influence on sintering process and the amount, composition and distribution of the secondary phase TiO1-xCx. The formation of TiO1-xCx is discussed in relation to volatile boron oxide, which reacts with the graphite die to produce CO gas, which further may cause transport of carbon into TiB2 during sintering before pore closure. Finally it was demonstrated that the density could be controlled by addition of carbon to the TiB2 powder.