Anionic diffusion in a soft crystal lattice of hybrid halide perovskites affects their stability, optoelectronic properties and the resulting device performance. The use of two-dimensional (2D) halide perovskites improves the chemical stability of perovskites and suppresses the intrinsic anionic diffusion in solid-state devices. Based on this strategy, devices with an enhanced stability and reduced hysteresis have been achieved. However, a fundamental understanding of the role of organic cations in inhibiting anionic diffusion across the perovskite-ligand interface is missing. Here we demonstrate the first quantitative investigation of the anionic interdiffusion across atomically flat 2D vertical heterojunctions. Interestingly, the halide diffusion does not follow the classical diffusion process. Instead, a 'quantized' layer-by-layer diffusion model is proposed to describe the behaviour of the anionic migration in 2D halide perovskites. Our results provide important insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform with an enhanced stability for heterostructure integration.