The potential of using carbon as a p-type dopant for InAlAs has already been recognized due to the achievable high hole concentration [1,2]. However, high doping levels are reached only for low growth teperatures (Tg below 600°C). These temperatures produce layers with poor optical quality as compared to those grown at higher temperatures, which can be detrimental for optoeletronic device. In this work we present crystal, transport and optical properties of such layers grown at different temperatures. We find that the lower Tg, the more efficient the carbon incorporation and its electrical activity are. This result indicates that carbon is incorporated in forms different from a shallow acceptor, as well. Carbon can also be incorporated as a shallow donor since it is an amphoteric dopant. However, this alone does not explain the transport results. The difference between the net free charge density determined from capacitance measurements indicates that a deep donor is also incorporated. Carbon most likely participates in the deep donor formation since the inferred deep donor concentration varies linearly with the carbon atomic density measured by SIMS. On the other hand, non- radiative deep levels are more efficiently incorporated as Tg is reduced degrading the photoluminescence characteristics. Such degration is independent of the carbon doping. Therefore, one concludes that the decrease in the photoluminescence efficiency cannot be related to the presence of the deep donor mentioned in the previous paragraph. To further probe the carbon electrical activity and its effect on the optical properties of the layers, the samples have been subjected to a heat-treatment. Annealing the samples increases the hole concentration, but neither affects the deep donor density nor improves the layers optical quality. In order to use carbon doped InAlAs in devices which simultaneously require good optical quality and high electrical activity of the layers, one should identify the deep donor involving carbon in order to try to reduce its concentration or even eliminate it, consequently improving the electrical activity of the layers. In such a way the layers can be grown at higher temperatures, adequate for an efficient photoluminescence emission. Theoretical calculations are being carried out to help with such identification. Another possibility is to use other arsine sources which crack at lower temperatures.