Abstract The crystal structure of a monoclinic poly-morph of methyl maleic acid (citraconic acid, polymorphII) was determined using single crystal synchrotron datafrom a small fragment extracted from the reagent bottle.The material crystallizes in space group P2 1 /c with unit cellparameters a = 8.109(2), b = 7.090(2), c = 10.709(3) A˚ ,b = 114.148(16) , V = 561.8(3) A˚ 3 , Z = 4, d c =1.538 g cm -3 . In this polymorph, the carboxylic groupwith the a-methyl group participates in cyclic hydrogenbonds with graph set symbol R 22 ð8Þ, while the other car-boxylic group participates in a hydrogen bonding patternrepresented by the C(4) graph set symbol. In contrast, in apreviously reported polymorph (polymorph I) obtained byslow evaporation from ethyl acetate, both carboxylicgroups participate in cyclic hydrogen bonding motifs.Thermal analysis suggests that the transformation ofpolymorph II into polymorph I is monotropic.Keywords Citraconic acid Polymorphism Single crystal synchrotron studyIntroductionThe subject of polymorphism continues to attract great dealof interest in the pharmaceutical industry [1, 2] because ofits scientific and economic impact and implications. Fromthe chemical point of view, it is certainly an interesting andintriguing phenomenon. Some compounds have shown to bepolymorphic after being known as monomorphic for manyyears after their discovery or preparation, such as the case ofmaleic acid [3, 4]. Controversy has usually accompaniedthese findings. For example, it was recently reported that atheoretically predicted polymorph of aspirin [5] had beenfinally encountered [6]. However, other authors later pos-tulated that the diffraction pattern of the proposed newpolymorph can be interpreted as a superposition of thepatterns of two intergrown domains which, in variableratios, are present in the structure of aspirin that we haveknown for all these years [7, 8]. In other instances, newpolymorphs of a single compound keep appearing, as is thecase of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophene-carbonitrile (ROY), with nine polymorphs reported up to2005 [9] reinforcing McCrone’s well known assertion that‘‘It is at least this author’s opinion that every compound hasdifferent polymorphicforms andthat,ingeneral,thenumberof forms known for a given compound is proportional to thetime and money spent in research on that compound’’ [10].As part of the work carried out for the synthesis of metalderivatives of unsaturated dicarboxylic acids at room tem-perature and under solvothermal conditions, we routinelycharacterize the reagents and reaction products by spectro-scopic, thermal, and X-ray diffraction techniques. Amongthe reagents, we have used methylmaleic acid (citraconicacid, Fig. 1). When characterizing this acid, we observedthat the powder diffraction pattern obtained for reagentgrade citraconic acid (purchased from Sigma–Aldrich) was