The limited penetration of cytotoxic drugs into tumors is a significant contributing factor to the limited effectiveness of cancer chemotherapy. Here we have shown that X-ray fluorescence microtomography is a suitable technique for imaging the distribution of Pt drugs within multicellular tumor spheroids, models of solid tumors. This has revealed that there is no significant difference between the distributions of the range of Pt(II) and Pt(IV) complexes investigated, suggesting that complexes that are more resistant to cellular uptake may be needed to ensure more complete distribution throughout a tumor.