Supramolecular organic dye–inorganic semiconductor nanocrystal assemblies are potentially useful in a broad range of technologies and applications, including photovoltaic systems, but the molecular basis of the adsorption of dye molecules onto the semiconductor surfaces remains poorly understood. Herein, we investigated the pH-dependent adsorption and conformational change of two cationic porphyrin stereoisomers [5,10-diphenyl-15,20-di([Formula: see text]-methyl-4-pyridyl)porphyrin (cis-DMPyP) and 5,15-diphenyl-10,20-di([Formula: see text]-methyl-4-pyridyl)porphyrin (trans-DMPyP)] on the tungsten(VI) oxide (WO[Formula: see text] colloid nanoparticle in aqueous media by means of UV-vis absorption spectroscopy. In accordance with the combination of a modified Langmuir adsorption model and Kasha’s exciton coupling model, the molecular orientation and stacking arrangement of DMPyP derivatives on the WO[Formula: see text] colloid surface are discussed in detail. In the trans-DMPyP/WO[Formula: see text] aqueous system, trans-DMPyP molecules adopted flat-on orientation with respect to the WO[Formula: see text] colloid surface and eventually formed head-to-tail [Formula: see text]-dimers regardless of pH conditions. cis-DMPyP molecules in the acidic system also lay flat-on and mainly formed [Formula: see text]-dimers on the WO[Formula: see text] colloid surface, whereas ones in the neutral system exhibited a dominant edge-on orientation and had a higher tendency to form face-to-face [Formula: see text]-dimers. Additionally, we have also convincingly demonstrated the pH-triggered switchable [Formula: see text]-stacking geometry of cis-DMPyP molecules from [Formula: see text]- to [Formula: see text]-dimer and vice versa on the WO[Formula: see text] colloid surface. Such findings will undoubtedly provide a pertinent guideline for the rational design of stimuli-responsive organic-inorganic materials.