A study of the reaction between ninhydrin and alanine has been conducted, taking into account that, adhered on the surface of a dry porous medium such as paper, a quasi-heterogenous reaction must occur. Instead of amino acids released from human sweat glands, aqueous solutions of alanine were taken to deliver a given amount of amino acid to the sample. The dye density, obtained after using a standard development process, could noticeably be increased by setting a drop of water on the dye dot, thus indicating that not all the alanine had been used for dye formation during the usually applied process. The incomplete reaction can be explained by the problem of bringing the reactants into contact with each other when both are in the solid phase in the porous surrounding. The temporary presence of water allows a re-orientation of the insoluble reactants. With fingerprints, an increase in both the rate of development and the final dye density could be obtained when the sample was post-treated after the developing process with the blank solvent, such that the background coloration could be decreased. The ideas presented in this paper may form the basis for a modification of developing processes with ninhydrin in order to increase the proportion of amino acids present (in the sample) used in dye formation without data loss. 7 figures and 17 references