Background Thermochemical ablation (TCA) is a minimally invasive therapy under development for hepatocellular carcinoma. TCA simultaneously delivers an acid (acetic acid, AcOH) and base (sodium hydroxide, NaOH) directly into the tumor, where the acid/base chemical reaction produces an exotherm that induces local ablation. However, AcOH and NaOH are not radiopaque, making monitoring TCA delivery difficult. Purpose We address the issue of image guidance for TCA by utilizing cesium hydroxide (CsOH) as a novel theranostic component of TCA that is detectable and quantifiable with dual‐energy CT (DECT). Materials and methods To quantify the minimum concentration of CsOH that can be positively identified by DECT, the limit of detection (LOD) was established in an elliptical phantom (Multi‐Energy CT Quality Assurance Phantom, Kyoto Kagaku, Kyoto, Japan) with two DECT technologies: a dual‐source system (SOMATOM Force, Siemens Healthineers, Forchheim, Germany) and a split‐filter, single‐source system (SOMATOM Edge, Siemens Healthineers). The dual‐energy ratio (DER) and LOD of CsOH were determined for each system. Cesium concentration quantification accuracy was evaluated in a gelatin phantom before quantitative mapping was performed in ex vivo models. Results On the dual‐source system, the DER and LOD were 2.94 and 1.36‐mM CsOH, respectively. For the split‐filter system, the DER and LOD were 1.41‐ and 6.11‐mM CsOH, respectively. The signal on cesium maps in phantoms tracked linearly with concentration (R2 = 0.99) on both systems with an RMSE of 2.56 and 6.72 on the dual‐source and split‐filter system, respectively. In ex vivo models, CsOH was detected following delivery of TCA at all concentrations. Conclusions DECT can be used to detect and quantify the concentration of cesium in phantom and ex vivo tissue models. When incorporated in TCA, CsOH performs as a theranostic agent for quantitative DECT image‐guidance.