The　present　study　compared　measured　heat　transfer　coefficients　for　supercritical　pressure　R134a　in　a　horizontal　internally　ribbed　tube　(d(in)　=　15.5　mm)　and　in　a　smooth　tube　(d(in)　=　16　mm)　for　mass　fluxes,　G,　from　100　to　350　kgm(-2)s(-1),　heat　fluxes,　q,　from　10　to　25　kWm(-2),　pressures,　P,　from　4.26　to　5　MPa　and　inlet　temperatures　from　70　to　105　degrees　C.　The　heat　transfer　coefficients,　buoyancy　effects　and　pressure　drop　are　compared　for　the　various　operating　conditions.　The　results　show　that　the　tube　wall　temperature　in　the　internally　ribbed　tube　varied　less　for　the　various　working　conditions　than　in　the　smooth　tube.　Comparisons　of　the　measured　heat　transfer　coefficients　for　all　4800　experimental　data　points　showed　that　the　heat　transfer　coefficients　in　the　internally　ribbed　tube　are　4.61　times　those　in　the　smooth　tube　on　the　top　and　1.40　times　those　in　the　smooth　tube　on　the　bottom.　The　buoyancy　threshold　in　the　internally　tube　is　higher　than　in　the　smooth　tube　for　both　the　Gr(q)/Gr(th)　and　Grb/Reb2.0　(rho　b/rho　w)x/d　criteria　which　shows　that　the　buoyancy　in　the　internally　ribbed　tube　is　suppressed.　Moreover,　the　pressure　drop　in　the　internally　ribbed　tubes　is　higher　than　in　the　smooth　tubes　for　the　same　working　conditions.