The ibuprofen primary binding site FA3–FA4 is located in domain III of human serum albumin (HSA), the secondary clefts FA2 and FA6 being sited in domains I and II. Here, the thermodynamics of ibuprofen binding to recombinant Asp1–Glu382 truncated HSA (tHSA)–heme-Fe(III) and nitrosylated tHSA–heme-Fe(II), encompassing domains I and II only, is reported.