A method is described for the analysis of thorium by neutron activation. The sample is brought into solution and quantitative extraction of233Pa is carried out in 1M HNO3 using 0.5M HTTA in benzene. Extraction mechanisms are discussed. The use of a low-power reactor (100 kWth) enables determination of thorium at a lower concentration limit of 50 ng for periods of irradiation, cooling and measurement of 3 hours, 24 hours and 300 seconds, respectively.