Despite numerous publications on picture varnishes, little information is available on their optical properties. Varnishes, however, greatly influence the appearance of paintings. Modern synthetic varnishes, used by restorers as replacements for unstable natural resin varnishes, do not produce the same amount of gloss and color saturation as traditional varnishes. Evidence is presented that molecular weight and refractive index are key factors in determining the optical properties of varnishes. Surface tension and hence molecular composition are of minor importance taking into consideration that varnishes of practical interest are all of relatively low polarity. Varnishes for Old Master paintings that aim at imitating the appearance of traditional picture varnishes should have low molecular weight, and a relatively high refractive index.