In this paper, we address a class of problems in unitary ensembles. Specifically, we study the probability that a gap symmetric about 0, i.e. (−a, a) is found in the Gaussian unitary ensembles (GUE) and the Jacobi unitary ensembles (JUE) (where in the JUE, we take the parameters α = β). By exploiting the even parity of the weight, a doubling of the interval to (a2,∞) for the GUE, and (a2, 1), for the (symmetric) JUE, shows that the gap probabilities maybe determined as the product of the smallest eigenvalue distributions of the LUE with parameter α = −1/2, and α = 1/2 and the (shifted) JUE with weights x1/2(1 − x)β and x−1/2(1 − x)β. The σ function, namely, the derivative of the log of the smallest eigenvalue distributions of the finite-n LUE or the JUE, satisfies the Jimbo–Miwa–Okamoto σ form of PV and PV I , although in the shift Jacobi case, with the weight xα(1 − x)β, the β parameter does not show up in the equation. We also obtain the asymptotic expansions for the smallest eigenvalue distributions of the Laguerre unitary and Jacobi unitary ensembles after appropriate double scalings, and obtained the constants in the asymptotic expansion of the gap probabilities, expressed in term of the Barnes G-function valuated at special point.