Positron annihilation measurements of natural brown type I diamonds revealed that they contain monovacancy-type defects and large vacancy clusters. The latter defect type was absent in clear diamonds. Both the intensity of the positron vacancy cluster lifetime and the S-parameter were found to increase with the intensity of brown colouration determined by optical absorption measurements. No correlation was found between the nitrogen content and the positron annihilation parameters. © 2013 Elsevier B.V.