In the SrO+VO 2 phase diagram there exists no SrV 4 O 9 phase corresponding to the CaV 4 O 9 phase in the CaO+VO 2 diagram and synthesis of SrV 4 O 9 has not been reported so far. We are successful in hydrothermal synthesis of the synthesis of SrV 4 O 9 : hydrothermal treatment of SrCl 2 +NaVO 3 +(CH 3 ) 4 NCl solutions above 3003C yielded light-green plate crystals identi5ed as SrV 4 O 9 . Single-crystal X-ray di4ractometry con5rmed the CaV 4 O 9 -type structure consisting of V 4 O 9 layers and interstitial Sr atoms: P4/n, a โซุโฌ 8.379(2) A s , c โซุโฌ 5.259(3) A s , and Z โซุโฌ 2. The re5nements based on 826 re6ections with I > 3 (I) converged to R โซุโฌ 0.039 and R w โซุโฌ 0.048. Temperature variation of magnetic susceptibility exhibits a low-dimensional feature of a broad maximum around 100 K, just like that of CaV 4 O 9 . Single crystals of CaV 4 O 9 were similarly grown in the hydrothermal CaCl 2 +NaVO 3 +(CH 3 ) 4 NCl system at 2803C and its single-crystal X-ray study was also made to compare with those of SrV 4 O 9 .