The measurement of surface pressure-area isotherms of short chain fatty acids on a Langmuir trough often shows irreproducible, large errors, in spite of high purity of materials, cleanliness of equipment and sophistication of routines. By controlling the timing during monolayer spreading, the spreading speed and the history and age of die subphase, these errors become systematic and reproducible. On the basis of myristio acid, where the effect is very large, and palmitic acid, for which the effect is far from negligible, it is shown that the governing effect is dissolution and saturation of the molecules in the subphase. This raises the question of metastability of such monolayers not only above the equilibrium spreading pressure but at any pressure. It also explains some, so far, ill-understood problems about Langmuir-Blodgett film fabrication and the measurement of equilibrium spreading pressures.