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We establish Holder estimates of second derivatives for a class of sub-elliptic partial differential operators in \({\mathbb{R}^{N}}\) of the kind $$\mathcal L=\sum_{i,j=1}^{m}a_{ij}(x)X_{i}X_{j}+X_{0},$$ where the Xj’s are smooth vector fields in \({\mathbb{R}^{N}}\), and aij is a uniformly elliptic matrix. It is assumed that the Xj’s satisfy homogeneity conditions with respect to a group of dilations δr which yield the existence of a composition law \({\circ}\) in \({\mathbb{R}^{N}}\) making the triplet \({\mathbb G=(\mathbb{R}^{N},\circ,\delta_{r})}\) an homogeneous Lie group on which the Xj’s are left translation invariant. The Holder norms are defined in terms of this composition law. The main tools used are the Taylor formula for smooth functions on \({\mathbb{G}}\), some properties of the corresponding Taylor polynomials, and an orthogonality theorem that extends to homogeneous Lie groups a classical theorem of Calderon and Zygmund in the Euclidean setting.