Abstract—Initial quantum yields for the formation of all four uracil dimers,anti h, t(I);anti h, h(II);syn h, h(III) and syn h, t (IV), have been quantitatively determined in aqueous degassed and air‐saturated solutions as well as in degassed and air‐saturated solutions of micellar sodium dodecyl sulfate (NaLS) and hexadecyltrimethylammonium bromide (CTAB). In aqueous solutions the order of dimer yields isIV>III>II>I. Percentages of dimerIIIdecreases from 18‐9 per cent in water to 2–7 per cent in micellar CTAB and to 6.0 per cent in micellar NaLS, but those for dimersIandIIincrease in the micellar