Recently, there is increasing interest in using butanol as a renewable component not only for gasoline but also for diesel fuels. This work investigates the effect of its concentration on diesel and biodiesel blends on the autoignition time, comparatively to equivalent blends with ethanol, in a constant-volume combustion chamber. The increase of alcohol content in diesel or biodiesel, led to an increase in autoignition times. Such increase was not linear but sharper for high alcohol contents, and was higher with ethanol than with butanol. For butanol blends, the increase in delay time was very similar when diesel or biodiesel were used. The maximum pressure during combustion decreases as the alcohol content was increased, especially in the case of ethanol, as a consequence of energy, chemical and dilution effects. However, for low ethanol or butanol concentrations, some increases in the pressure peaks were observed by a combination of compensating effects: increase in the amount of premixed combustion and increase in the flame speed. For 10% v/v alcohol blends in diesel or biodiesel, the delay times decreased as both the initial pressure and the initial temperature were increased, the latter effect being slightly higher with biodiesel compared to diesel