We address the problem of the comparative environmental fate of a pesticide, chlordecone (CLD), and a related compound, chlordecone-5b-hydro (CLD-5b-hydro). We used a large database including data from two types of contaminated volcanic soils, andosol and nitisol, and thirteen crops grown in the French West Indies in historically polluted soils. We performed in-depth statistical analysis of the effect of different parameters (soil type, crop, organ, etc.) on the ratio of CLD-5b-hydro to CLD in both soils and plants. The environmental fate of the two compounds differed depending on the type of soil. Proportionally, more CLD-5b-hydro than CLD was measured in nitisols than in andosols. Compared to CLD, we also found a preferential transfer of CLD-5b-hydro from the soil to the plant. Finally, mobilization of the two compounds differed according to the species of crop but also within the plant, with increasing ratios from the roots to the top of the plant. The properties of the compound played a key role in the underlying processes. Because CLD-5b-hydro is more soluble in water and has a lower Kow than CLD, CLD-5b-hydro (1) was more easily absorbed from soils by plants, (2) was less adsorbed onto plant tissues and (3) was transported in greater quantities through the transpiration stream. Due to the amounts of CLD-5b-hydro we measured in some plant parts such as cucurbit fruits, an assessment of the toxicity of this CLD monodechlorinated product is recommended.