A general route to pyrazole derivatives involves reaction of an arylhydrazine, ArNHNH 2 , with a β-dicarbonyl compound, R/COCH 2 COX.. This reaction provides initially a hydrazone derivative, RNHN=CR/CH 2 COX, I ( Fig. 1 ), which can be isolated but which readily undergoes cyclization to a pyrazone derivative, II ( Fig.1 ), (Nef, 1891; Katritzky et al., 1997; Wardell et al., 2007; de Lima et al., 2010) . Equilibrium involving tautomers of II in solution have been variously studied using NMR and IR spectroscopy and ab initio calculations. (Feeney et al., 1970; Hawkes et al., 1977; Freyer et al., 1983; Dardonville et al., 1998; Kleinpeter & Koch, 2001) . Crystal structures of various pyrazone compounds of forms IIa, IIb and IIc have been reported (see for example, Bechtel et al., 1973a; Bechtel et al., 1973b; Chmutova et al., 2001; Wardell et al., 2007; Gallardo et al., 2009; Ding & Zhao, 2010) . In continuation of recent studies (Wardell et al., 2012) , herein, the isolation the title compound [i.e. form IIc ( Fig. 1) ] from the reaction between PhNHNH 2 and PhCOCH 2 CO 2 Et in EtOH is described as is its crystal structure. The same tautomer was also isolated in the reaction between PhNHNH 2 and PhCOCH 2 CONHPh in EtOH.