Isothermal stability field diagrams for Ln−O−Cl systems (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) are developed by taking partial pressures of volatile components oxygen and chlorine as variables. Thermodynamic properties of all the oxides and trichlorides (LnCl$_3$) are available in the literature. However, data for oxychlorides (LnOCl) and dichlorides (LnCl$_2$) are limited. Based on systematic trends in stability of these compounds across the lanthanide series, missing data are estimated to construct the diagrams for 13 Ln−O−Cl systems at 1000 K. All the lanthanide elements form stable LnCl$_3$ and LnOCl. Dichlorides of Nd, Sm, Eu, Dy, Tm and Yb are stable. For systems in which dichlorides are unstable (Ln = La, Ce, Pr, Gd, Tb, Ho, Er), the LnOCl is in equilibrium with the metal (Ln) and the stability field of LnOCl is sandwiched between those of oxides and trichlorides. Stability field diagrams of lanthanide systems forming stable LnCl$_2$ are of two kinds: in the first kind (Ln = Nd,Dy) the stability fields of Ln and LnOCl are in contact and the stability field of LnOCl separates the fields of chlorides and oxides. In diagrams of the second kind (Ln = Sm, Eu, Tm, Yb) there is a direct equilibrium between the oxides and dichlorides at low partial pressures of oxygen and chlorine. There is no contact between the stability fields of Ln and LnOCl; the stability field of LnOCl intervenes between the oxide and chloride phases only at higher partial pressures.