Results of theoretical and experimental investigations of the thermal behaviour of InGa-AsP/InP laser diodes with a ridge-waveguide structure are presented. It is shown that, in contrast to GaAlAs/GaAs laser diodes, most of the heat flux is carried through the InP-substrate, less than 1/4 through the ridge. The temperature rise in the active region was determined and the thermal resistance calculated for various structure and bond parameters. The theoretical and experimental results fit very well. The change of the thermal resistance compared to a norm value of 67 K/W with variation of structure parameters is discussed. It is strongly affected by the device length, the ridge width and the bonding parameters.