The binding of chlorin p(6) with Cremophor EL (CrEL) at pH 5 and pH 7 has been monitored by fluorescence techniques. Upon binding with the drug delivery agent, the fluorescent species are found to be one and the same at both pH values. This is explained by a hindrance to protonation of chlorin p(6) trianions by Cremophor EL. Surprisingly, it is the more hydrophilic species between the two possible chlorin p(6) ions that is found to get bound preferentially with the hydrophobic delivery medium. (c) 2005 Elsevier B.V. All rights reserved.