To find an explanation for the reported positive effect of a dimethyl sulfoxide (DMSO) treatment on the performance of the bone scan agent technetium-(tin)-ethane-1-hydroxy-1, 1-diphosphonate, we compared the composition of the agent, prepared with and without treatment with DMSO by using high performance ion-pair chromatography (IPC). The preparation obtained with the DMSO treatment appeared to contain a larger fraction of large and highly charged polynuclear complexes than the preparation without the DMSO treatment. According to experiments by other investigators the smaller 99mTc(Sn)EHDP complexes (early eluting components in IPC) give lower bone/blood ratios than the larger ones. The results presented in this paper show that the explanation for the effect of the DMSO treatment may be that the Sn-EHDP complexes which are removed by extraction with DMSO, give relatively small 99mTc(Sn)EHDP complexes. Without these small complexes a superior bone scan agent is obtained. From experiments in which an excess of 99TcO4 over Sn(II) was used, it was concluded that at least one of the technetium complexes contains Sn(IV). On the other hand, the absence of Sn from a late eluting technetium complex was proven.