A variety of research studies have been investigating the compatibility of primary alcohols as alternative fuels for diesel engines, in view of depleting crude oil reserves and resulting environmental degradation. Pollutants are formed in the engine combustion chamber and are mostly controlled using exhaust gas after-treatment technologies. However, active characterization of harmful pollutants such as soot and oxides of nitrogen (NOx) can be done in situ, at the locations of their formation in the engine combustion chamber by employing optical diagnostic techniques. In this study, the high temperature industrial endoscopy technique was adapted to a single cylinder diesel engine for in situ optical diagnostics of engine combustion for diesel and diesohols (diesel alcohol blends) such as E20 and M20 fuels. In-cylinder combustion images were acquired by the endoscopic system and processed further for spatial soot and flame temperature distributions. Spatial distribution of in-cylinder soot was evaluated from "R intensity" values, and flame temperature distribution was evaluated by the correlated color temperature (CCT) method. The image distortions because of endoscopic lens were removed for the first time in this study. Luminous regions in the endoscopic images indicated soot radiations and therefore the locations where the soot is formed inside the combustion chamber, whereas the soot radiation temperature represented the flame temperature for diesel combustion. Both diesohol blends showed superior characteristics in terms of combustion and emissions compared to baseline mineral diesel.