Plants are newly discovered sources of important biopolymers called humic acids (HA). Purified HA isolation from plants and other sources begins with sequential Soxhlet extraction with diethylether, acetone, ethanol and dioxane. We have investigated the characteristics of these steps in the isolation of HA from the brown alga Pilayella littoralis. The mass balance of the total HA isolation procedure and the time dependences of each step are reported. Experimental extraction variables included the solvent cycle time, temperature, age since harvesting of the Pilayella source material and the length to diameter ratio of the source extracted. Non-humic constituent extraction from Pilayella does not depend on solvent diffusion in or out of the source matrix. Extraction rates are determined by rapid, non-ideal equilibria or zero order kinetic processes, depending on the solvent, the duration of extraction and the age of the Pilayella sample. The equilibria and kinetically controlled steps have small temperature dependences. The data will assist HA isolation from other sources. Since non-humic substances are removed, we endorse reflux solvent Soxhlet pre-extraction in the isolation of purified HA from plants and other sources.