The surface acidity of zirconium phosphate at different stages of dehydration and heat treatments has been studied by Fourier-transform infrared spectroscopy of adsorbed pyridine, acetonitrile and acetone and by catalytic activity in the isomerization of but-1-ene. Brönsted-acidic surface POH and P(OH)₂ groups are identified [v(OH)= 3670–3660 and 3600 cm^–1, respectively] whose strength increases slightly on bulk dehydration. They are thought to be responsible for the activity in but-1-ene isomerization, which also increases during condensation to pyrophosphate. Lewis-acidic sites of medium-high strength have also been found, and responsible for the formation of chemisorbed forms of pyridine (v_8a= 1610 cm^–1), acetonitrile [v(CN) Fermi resonance doublet at 2322 and 2295 cm^–1] and acetone [v(CO) 1684 cm^–1]. Surface ZrOH groups are also detected on the layered ZrP₂O₇ surface. The results illustrate the role of exposed planes, both parallel and perpendicular to the layered structure.