The oxygen isotopic composition (Δ18Oaragonite) of coral aragonite is dependent upon two parameters, temperature and water oxygen isotopic composition. This tracer allows temperature reconstruction in tropical zones. Different published Δ18O:temperature relationships show a slope derived from the regression Δ18O against T(¿C) which varies between -0.16 and -0.23 for Porites genus. This implies greater uncertainty for paleoclimatic reconstruction. The calibrations were generated differently. They were obtained from several colonies, collected in different locations or the formula was derived from the same coral head following seasonal variations of temperature. The relationships took into account of Δ18OSw fluctuation or Δ18OSw were estimated from salinity or were not considered. We show by using isotopic data from Weber and Woodhead (1972) and assessing the Δ18OSw value by two different methods that a thermodynamically significant formula is generated. ¿ 2001 American Geophysical Union<