The recombination centre that emerges in boron- and oxygen-containing silicon was thought to be a complex of a substitutional boron atom Bs and an oxygen dimer O2. However in material co-doped with boron and phosphorus, the degradation parameters were reported to correlate with the hole concentration p rather than with the boron concentration. In the present work, the temperature dependence of the Hall Effect was measured in co-doped and reference samples and the concentrations of isolated acceptors Na and of donors Nd were deduced. The value of Na was found to be substantially larger than p and close to the expected total concentrations of boron. This result clearly shows that the reported correlation of the degradation with p implies a lack of correlation with Na. Such a behaviour is accounted for by a model based on formation of BiO 2 complexes (involving an interstitial boron atom Bi rather than Bs): the grown-in concentration of this species is proportional to p and independent of Na.