As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI 3 -based devices and heterostructures is challenging due to its extreme instability under ambient conditions. Here, we present Raman characterization of CrI 3 and demonstrate that the main degradation pathway of CrI 3 is the photocatalytic substitution of iodine by water. While simple encapsulation by Al 2 O 3 , PMMA, and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing light exposure markedly improves stability, and CrI 3 sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of the CrI 3 /graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene. \ 2018 American Chemical Society.