Readme for data used in the article 'The effect of haematocrit on the mid-infrared refractive index of blood plasma,' published by Biosensors. This readme was created in November 2021. Dataset DOI: https://doi.org/10.5258/SOTON/D1621 Article DOI: https://doi.org/10.3390/bios11110417 This data is used in the article 'The effect of haematocrit on the mid-infrared refractive index of blood plasma,' published by Biosensors. The data contained in data.xlsx are those used to plot the figures in the article. Measurement data were collected by ATR-FTIR spectroscopy at the University of Southampton during December 2019. Full methodological details can be found in the article. The XY data for each figure are contained in separate worksheets within data.xlsx. Each dataset is labelled with its name and unit. For plots with several spectral traces with respect to wavenumber, each trace is sampled at identical wavenumbers so wavenumber is only listed once. Briefly, each figure shows: Figure 1: Absorbance spectra of (a) DI water, plasma and whole blood with haematocrit in the range 20–70%, (b) plasma and whole blood with haematocrit in the range 20–70% over a more limited frequency range (1370–1570 cm−1), and (c) absorbance at 1541 cm−1 with respect to haematocrit. Figure 2: Empirical effective penetration depth deff calculated from the measured absorbance and literature k values of water. The dashed trace at wavenumbers > 3700 cm^-1 show the region where deff has been calculated from a ratio where both quantities are approximately equal to zero so cannot be relied upon. Figure 3: Imaginary part of refractive index spectra k for water and whole blood with haematocrit in the range 20 – 70%. Figure 4: Real part of refractive index spectra n for water and whole blood with haematocrit in the range 20 – 70%. Figure 5: Error in (a) real and (c) imaginary parts of plasma refractive index due to haematocrit in the range 20 – 70%. (b) shows the maximum error in n, which occurs at 1560 cm^-1, with respect to haematocrit; (d) shows the corresponding behaviour for k, which occurs at 1541 cm^-1. The data may be reused under Creative Common Attribution v4.0. Authors David J Rowe (1), Daniel R Owens (2, 3), Suzanne L Parker (4), Saul N Faust (2, 3), James S Wilkinson (1) and Goran Z Mashanovich (1) Affiliations (1) Zepler Institute for Photonics and Nanoelectronics University of Southampton Highfield Southampton SO17 1BJ UK (2) Faculty of Medicine and Institute for Life Sciences University of Southampton Tremona Road Southampton SO16 6YD UK (3) NIHR Southampton Clinical Research Facility and Biomedical Research Centre University Hospital Southampton NHS Foundation Trust Tremona Road Southampton SO16 6YD UK (4) University of Queensland Centre for Clinical Research The University of Queensland Herston QLD 4029 Australia