The kinetics of transesterification in blends of thermotropic copoly(oxybenzoate-ethylene terephthalate) and polycarbonate at melt state were analysed quantitatively with C-13 nuclear magnetic resonance, and were then modelled by a phenomenological equation. It was found that the activation energy for the transesterification reaction between the poly(oxybenzoate) segment in copoly(oxybenzoate-ethylene terephthalate) and polycarbonate was more than twice that for the transesterification reaction between the poly(ethylene terephthalate) segment and polycarbonate (i.e. 94.6 versus 41.8 kcal/mol). This is probably due to the stronger chemical bond in oxybenzoate-oxybenzoate that in ethylene-terephthalate. Additionally, owing to the rigid-rod nature of the poly(oxybenzoate) segment, the frequency factor in the transesterification reaction in poly(oxybenzoate) segment and polycarbonate is much higher than that in poly(ethylene terephthalate) segment and polycarbonate (i.e. 10(36) versus 10(14) min(-1)).