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We made a theoretical investigation on the top-hat-beam Z scan when the nonlinear media possess third- and fifth-order nonlinearities simultaneously. A high-accuracy method was proposed to allow the determination of the nonlinear refraction coefficients related to the third- and fifth-order effects. We also reconfirmed the common procedure for dividing the contributions of the third- and fifth-order effects to the nonlinearity in the top-hat-beam Z-scan measurements; the results indicated that the common procedure can be still employed for the determination of the third-order nonlinearity, whereas it is inapplicable for the fifth-order nonlinearity. We investigated experimentally, as a test, the nonlinear refraction of a Disperse Yellow 7 thin film by using the top-hat-beam Z-scan technique, with 35-ps-duration pulses at a 1064-nm wavelength.