. Knowl ed.ge of t he cOITlpos ition of oil s ll sed in oil-exte nd ed rubber is useful in s peeif.,·ing ot! types and til understand In g t he proper ties of t he oil-rub ber master batches. In th is work , elucidation of structure has bee n attempted by a compari so n of infrared spectra betwee n 2 and 15 microns of two se ri es of oil fractions obtaincd by bas icall y d iffere nt me thods. FractIOns of a number of differe nt oils or oil dist illates were studied. The spectra ,,"e re com pared wit h reference to t he method of separat io n and in some cases t o t he phys ical and chen1ical propert ies of t h e fractions. Ther e are marked s imilarities between ccrtain of the fraction s se parated by t he two methods. However, t he s tud~' indicates a variabi lity of stryctural types fro m one oil to another. This is esp eciall y t ru e for t he more p olar f ractio ns. It lss uggested that t hi s va rab ili ty, especially with res pect t o polar groups a nd other structures lead lllg to act lv" h yd rogens, could accou nt for t he obse rv ed variabili ty in t he aging propert ies of master batc hes co ntaining difi'ere nt oils or oil fraction s.Spectra were a lso obtain ed of a few t~' pica l fr actio ns in t he ul t rav iolet range bet\\'ee n 2 10 a nd 340 mi ll imicron s a nd in the far infrared be l\\'ee n 15 and 40 m icro ns.