The distributions and abundances of small organics in protoplanetary disks are potentially powerful probes of disk physics and chemistry . HNC is a common probe of dense interstellar regions and the target of this study . We use the Submillimeter Array ( SMA ) to observe HNC 3–2 towards the protoplanetary disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296 . HNC is detected toward both disks , constituting the first spatially resolved observations of HNC in disks . We also present SMA observations of HCN 3–2 , and IRAM 30m observations of HCN and HNC 1–0 toward HD 163296 . The disk-averaged HNC/HCN emission ratio is 0.1–0.2 toward both disks . Toward TW Hya , the HNC emission is confined to a ring . The varying HNC abundance in the TW Hya disk demonstrates that HNC chemistry is strongly linked to the disk physical structure . In particular , the inner rim of the HNC ring can be explained by efficient destruction of HNC at elevated temperatures , similar to what is observed in the ISM . To realize the full potential of HNC as a disk tracer requires , however , a combination of high SNR spatially resolved observations of HNC and HCN , and disk specific HNC chemical modeling .