IntroductionRecently, on-line process analysis based on near-infrared (NIR) spectroscopy has become a popular technology for many chemical and polymer industries. 1-31 An on-line monitoring system with optical fiber based on NIR spectroscopy is very suitable for use in a hazardous area. In addition to the above advantages, NIR spectroscopy can be applied to samples in various physical states, shapes and thicknesses. [1][2][3][4][5] Optical-fiber NIR spectroscopy has the possibility of allowing direct polymerization analyses, real-time monitoring of physical and/or chemical properties of molten polymers, and the quality control of polymers. Many researchers have been involved in on-line polymer process analyses by optical-fiber NIR spectroscopy, and have produced numerous publications. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][23][24][25][26][27][28][29][30][31] For realizing NIR on-line monitoring for molten polymers, further developments have been studied from the standpoints of hardware and software.Especially, from the software standpoint, robust calibration models, where changes in predicted values from on-line spectra due to various kinds of process disturbances are very small, must be developed. Moreover, it has been expected to develop precise calibration models using sample data and spectra with small size because of savings in costs and labor.We have started a series of studies to solve some of the abovementioned software problems. [16][17][18][19][20][21][22] In our first study of the series, we investigated the on-line monitoring of the density of linear low density polyethylene (LLDPE) in a real plant by NIR spectroscopy and chemometrics. 16,17 In a second study, predicting the ethylene (C2) content in melt random and block polypropylene (RPP and BPP) based on the NIR spectra of the molten polymer system was investigated. 18 A third study was concerned with the influence of a change in the sample temperature on predicting the C2 content in the melt state RPP and BPP and its compensation method. 19 In a fourth study, the possibility of improving calibration models for the vinylacetate (VA) concentration in melt-state ethylene-vinylacetate copolymers (EVA) for an on-line monitoring was reported. 20 A fifth study described a practical calibration correction method for an on-line NIR molten polymer system. 21 In a sixth study, 22 variations in the predicted values from the NIR spectra of samples in vials by using a calibration model developed from spectra of samples in vials were investigated, and modified MSC was proposed as a method for the continuous monitoring of the path-length in vials.The purpose of the present study is to demonstrate the simultaneous prediction of the C2 content and the melt temperature in melt RPP and BPP, and to investigate the effect of a modified MSC to variations in the predicted C2 content in the melt PP from spectra measured at different temperatures from that measured spectra set for the calibration model.The C2 content measurement in RPP and BPP is one of...