An experimental study of transition from steady laminar to chaotic flow in a horizontal annulus between concentric cylinders is conducted for 0.90×105[les ]RaL[les ]3.37×105. Qualitative information on the averaged thermal field is obtained by an interferometric method, whereas the kinematics are visualized by using smoke lines. The transition from steady to unsteady mono- and multi-periodic regimes is accurately studied by a CTA and CCA combined hot-wire technique. Different data analyses are performed in order to differentiate the forms of transition.