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A major difference between arterial‐spin‐labeling MRI and gold‐standard radiotracer blood flow methods is that the compartment localization of the labeled spins in the arterial‐spin‐labeling image is often ambiguous, which may affect the quantification of cerebral blood flow. In this study, we aim to probe whether the spins are located in the vascular system or tissue by using T2 of the arterial‐spin‐labeling signal as a marker. We combined two recently developed techniques, pseudo‐continuous arterial spin labeling and T2‐Relaxation‐Under‐Spin‐Tagging, to determine the T2 of the labeled spins at multiple postlabeling delay times. Our data suggest that the labeled spins first showed the T2 of arterial blood followed by gradually approaching and stabilizing at the tissue T2. The T2 values did not decrease further toward the venous T2. By fitting the experimental data to a two‐compartment model, we estimated …