Refractive index dispersion of thin films of deoxyribonucleic acid (DNA)-surfactant (CTMA) and doped DNA-CTMA-Nile Blue (NB) (5 w%) complexes are reported and discussed. The thermal variation of the thin film thickness was measured by ellipsometry. The DNA-CTMA complex exhibits a rarely observed negative thermal expansion between room temperature and ca. 85 degrees C. Doping with NB decreases the temperature range of negative thermal expansion to between room temperature and ca. 70 degrees C. The dopping by 5 w% of NB lowers the absolute value of negative thermal expansion of DNA-CTMA system from 5% to 2% within this temperature range.