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Abstract Two approaches have been adopted in attempting to determine the strength of the one-phonon infrared absorption by the B nitrogen aggregate in diamond. On the one hand, we have heat treated purely type IaA specimens with known infrared absorption strengths (and thus also known nitrogen concentrations) to bring about partial aggregation of the A centres to B centres, monitoring changes in the strengths of the A, and resultant B, one-phonon components by decomposition of the ensuing infrared spectra. Secondly, we have made direct chemical assays of nitrogen in diamonds that show dominant B absorption features. This approach is complicated because diamonds in which the B aggregate is the only point defect also contain extended defects that may or may not involve nitrogen. These defects may or may not, in turn, contribute to absorption in the one-phonon region. Nevertheless, the nitrogen concentrations have been measured for two distinct groups of diamonds that both uncritically might at first glance be typed together as ‘IaB’. By taking into account subtle differences between the infrared spectra of these groups, and making allowance for what is known from electron microscopical observations about their defect contents, it is possible, despite certain experimental indeterminacies, to arrive at a reasonably consistent interpretation of the measurements. A value of 79.4 ± 8 at.ppm/cm−1 is suggested for the absorption strength, at 1282 wavenumbers, of the B nitrogen aggregate.