This thesis is concerned with measurements of alternating field losses in cylindrical rods of niobium in its superconducting state. Six samples have been investigated each with a different surface condition, Magnetization measurements of the samples have been studied which show the effect of the surface condition on the first critical magnetic field H[sub]c₁. The effect of the surface on flux pinning are discussed. Power losses have been studied experimentally for single crystal niobium and for polycrystalline niobium each with three different surface conditions in order to study the effect of the surface properties on these ac losses. A wattmeter technique was developed for measurements of power losses in the samples. Experiments were conducted over a range of applied fields at 50 Hz and at 4.2 K. This technique will be described together with the experimental results. The results will be compared with predictions from a modified critical state theory and their significance discussed. The critical current density has been measured for each sample using a method based on the measurements of the power dissipation in a small modulating field, in the presence of a steady bias field. The technique will be described together with experimental results. The shielding field △H in the mixed state has been studied for the samples. The technique and the results will be discussed for each sample.