The strain in a porous Si layer formed by anodization of a crystallinep+‐Si(100) wafer in a HF electrolyte was measured by double‐crystal x‐ray diffractometry. The perpendicular strain in the as‐formed porous Si layer is ∼10−3. The parallel strain is not measurable (<10−4). Upon annealing in vacuum from 200–800 °C, at elevated temperature, the perpendicular strain decreases while the parallel strain remains zero. Subsequently, the perpendicular strain of the annealed sample increases by amounts that depend on the ambient. The observed changes of strain in porous Si layers are explained as being mainly caused by modification of the stress in the native oxide layer due to desorption and absorption of gas. We propose that the strain in porous Si is the net result of stress induced by surface tension, oxide formation, and exchange of gas between the oxide and ambient.