Interferometric measurements of the linear thermal‐expansion coefficients &agr;∥ and &agr;⊥ in the ``c'' and ``a'' axial directions of well‐oriented pyrolytic graphite between approximately 20° and 270°K are reported. Gru¨neisen parameters &ggr;∥ and&ggr;⊥, defined for strain coordinates parallel and perpendicular to the ``c'' axis, respectively, have been calculated from the present thermal expansion coefficients and related data. Between 30° and 270°K &ggr;∥ is positive, indicating the preponderance of longitudinal modes of vibration, while&ggr;⊥is negative, indicating the predominance of transverse modes. The quasiharmonic approximation has been applied to calculate the characteristic temperatures &thgr;(n) corresponding to the maximum frequencies &ohgr;D(n) of the Debye distributions having the samenth moments⟨&ohgr;n⟩as the specimens, which vary between approximately 420° and 2270°K asnincreases from −3 to +6. The dimensional dependence of the moments, defined by &ggr;(n) =&Sgr;&ggr;j&ohgr;jn/&Sgr;&ohgr;jncorresponding to &ggr;∥(n) and&ggr;⊥(n), and their variations withn, are consistent with the existence of low‐frequency modes of vibration between planes of atoms perpendicular to the ``c'' axis, and high‐frequency modes within the planes, a result supported by calculations of the rms displacements of the atoms.