Mixtures of isotactic bacterial poly(3-hydroxy butyrate) (PHB) with di-n-butyl phthalate (DBP) in the whole composition range are investigated by DSC. The glass transition (T-g) and melting temperature (T-m) of PHB/DBP mixtures regularly decrease with composition. Analysis of the melting point depression in terms of the Flory-Huggins theory yields the value DeltaH(u) = 11 kJ/mol for the melting enthalpy of fully crystalline PHB, in excellent agreement with earlier results. The isothermal crystallization of PHB/DBP mixtures with DBP volume fraction from 0 to 0.27 is investigated over a broad temperature range (from 30 to 130 degreesC). The spherulite growth rate curves (G vs T-c) shift to lower temperatures with increasing DBP content, following the Tg and T. changes. The maximum growth rate (G(max)) does not change with composition in the range explored. Analysis of the isothermal crystallization results by the Boon-Azcue modification of the Lauritzen-Hoffman treatment yields an estimate of the product of the crystal surface free energies sigmasigma(e) = 12.9 x 10(-12) J(2)/cm(4) that agrees with literature data and does not change with concentration in the range of DBP contents investigated.