The thermal behavior of visible AlGaInP-GaInP ridge laser diodes is investigated numerically and experimentally. It is shown that various parameters critically influence the thermal resistance R of such devices. R is inversely proportional to the thermal conductivity of the heatsink. A substantial improvement in R-quite larger than in the AlGaAs system-is achieved for junction-side-down mounting compared to junction-side-up. R depends strongly on the width w of the ridge and this effect is different for junction-side-up or down mounting. In the first case, R is similar to log (w) and in the second, R is similar to 1/w. The thickness of the soldering material is a sensitive parameter which may increase the value of R by up to 15 K/W. On the other hand, for junction-side-up mounted devices, the top metallization layer has a very favorable effect: a 1 mum thick gold layer reduces R by 30%. The dynamics of thermal phenomena is also studied. It is shown that when a laser is switched on the steady state is reached in the ms time range. Finally, our experimental results show a very good agreement with our numerical data.