5-Hydroxymethylcytidine (hm$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$C) was recently identified as a direct metabolite of m$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$C in RNA. We investigated the stability of hm$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$C in human cells using bio-isotopologues and LC-MS/HRMS. This has led to the discovery of a second oxidative metabolite of m$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$C in RNA, namely 2′-$0$-methyl-5-hydroxymethylcytidine (hm$\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}$C$m$). Subsequent quantitative analysis of total RNA from higher organisms revealed varying levels and TET-independent formation of this new RNA modification.