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Background/purpose: For medical and dermatological researchers, it is important to realize the molecular dynamics and its control in the stratum corneum (SC) of human skin, which may be related to some skin abnormalities such as atopic dermatitis and skin pruritus. We have tried to analyze the periodic molecular dynamics of the outermost layers of SC in vivo. Methods: We measured the skin surface molecules of human face in situ non-invasively using a Fourier-transform infrared (FTIR) spectroscopy system attached with a newly designed attenuated total reflection (ATR) probe. The water-extracted components from the SC were also analyzed using mass spectrometry, an enzymatic assay and high-performance liquid chromatography characterization. Results: The infrared spectral changes of some components on the face skin at around 1000–1200 cm−1 with circa-monthly rhythms were observed when monitored for 10 months, and the components also showed a seasonal change. The analysis of different FTIR spectrum of the changeable components with circa-monthly rhythm suggested the presence of a lactate compound. The presence of magnesium lactate in a conjugated form was detected in the water extract of SC. Conclusion: We demonstrate that the periodically changed components of the human face skin contained magnesium lactate conjugate as a major component. [ABSTRACT FROM AUTHOR]