Rhizophora mangle L. bark aqueous extract has antimicrobial, wound healing and antiulcerogenic properties. These properties could be associated with its chemical composition. To test this hypothesis, gravimetric, colorimetric, gas chromatography techniques were used to determine the preliminary chemical composition of this extract. Sephadex LH-20 Exclusion Chromatography was used by the fractionation of total extract and fractionation of low molecular weight polyphenols by liquid/liquid extraction. High Performance Liquid Chromatography was used to perform the composition in this low molecular weigh polyphenols fraction. The extract presented polyphenolic structures (54.78%) and other structural components (45.22%). Polymeric tannins were the major polyphenolic component (80%) and 20% were hydrolysable tannins. Epicatechin, catechin, chlorogenic acid, gallic acid and ellagic acid were monomeric structures determined in this extract. Phytosterols (0.0285%): stigmasterol, beta-sitosterol and campesterol were also present.