The structure of calcium arachidate Langmuir-Blodgett monolayers that were prepared at different pH in the subphase, were studied using vibrational sum frequency spectroscopy with a focus on testing their stability against melting upon heating. The pH controls the degree of deprotonation of the arachidic acid molecules. We find that at pH less than 9.5 for which the films are slightly less densely packed, they are also significantly less well ordered. Moreover, these films also develop gauche defects much more rapidly upon heating. Deriving an apparent activation energy for defect formation we find that it is 25% and 60% smaller for pH 7 and 4, respectively, when compared to pH 9.5. If prepared at pH 4, the film looses order already at 350 K. (c) 2017 Elsevier B.V. All rights reserved.