Doping of diamond by in-diffusion of lithium in natural type Ia and IIa single crystal diamond plates has been investigated. The lithium incorporation was realized by direct in-diffusion from a Li₂O source at temps. between 400 and 650^øC. Using secondary ion mass spectroscopy (SIMS) depth profiling, high concns. of lithium, up to a level of 8.0 *10²¹ cm^-3 and gradually decreasing to 1.0 *10¹⁹ cm^-3 at a depth of 0.5 mm below the surface, were measured. From these profiles the diffusion const. of lithium in diamond was detd. to be DLi(T) = 2 ï 10-10 exp (-0.9 eV/kT) cm² s^-1. The activation energy for diffusion of 0.9 ñ 0.3 eV is in agreement with the theor. literature value of 0.85 eV. Cathodoluminescence spectroscopy showed that the blue band-A luminescence, which is characteristic for natural type IIa diamond, is largely quenched by the presence of lithium. A shift of the max. luminescence intensity towards higher energy values has been found. From photothermal deflection spectroscopy and optical transmission measurements no new optically active defect is obsd. after lithium in-diffusion. However, these measurements indicated the occurrence of a large concn. of light scattering centers, the size of which is 10 nm or less. This suggests that lithium is clustered in the crystal vol. just below the specimen surface and therefore is not elec. active.