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We have investigated the photophysical properties and intracellular behaviour of a series of hydrophilic conjugated porphyrin dimers. All the dimers exhibit intense linear absorption at 650–800 nm and high singlet oxygen quantum yields (0.5–0.9 in methanol), as required for an efficient sensitiser for photodynamic therapy (PDT). They also exhibit fluorescence at 700–800 nm, with fluorescence quantum yields of up to 0.13 in methanol, and show extremely large two-photon absorption maxima of 8,000–17,000 GM in the near-IR. The dimers aggregate in aqueous solution, but aggregation is reduced by binding to bovine serum albumin (BSA), as manifested by an increase in fluorescence intensity and a sharpening in the emission bands. This process can be regarded as a model for the interaction with proteins under physiological conditions. Confocal fluorescence microscopy of live cells was used to monitor the rate …