We report a study of preactivation in the heterogeneous nucleation of ice by Langmuir films adsorbed at the surface Of water droplets. The Langmuir films consisted of aliphatic alcohols with 21, 23, or 25 carbon atoms per molecules. We find that the mean freezing temperature T*(f) of the water droplets decreases abruptly as the preparation temperature T-prep is increased. T-prep is the highest temperature to which the droplets are heated since the immediately preceding freezing event. Analysis of the distributions of freezing temperatures yields the dependence of thermodynamic parameters on T-prep, allowing us to identify the role of thermal history separately in the energy barrier and the attempt frequency for nucleation. We find that the change in T*(f) is exclusively due to a change in the energy barrier for nucleation and that all preactivation characteristics are a strong function of chain length. We find good agreement between our results and those expected from models for preactivation which rely on the expected properties of icelike ordered water near the nucleating agent. We also discuss the possibility that structural changes in the Langmuir film play a role in preactivation. We discuss future experimental directions for this system, now that preactivation behavior has been established.