A series of aromatic aldehydes have successfully been reduced by N-methylacridan AcH2 in the presence of perchloric acid (HClO4) in a mixture of acetonitrile and acetic acid (MeCN/MeCOOH, 4:1 v v) at 323 K. The effects of HClO4 as well as Mg(ClO4)2 on hydride-transfer reactions from AcH2 to a series of p-benzoquinone derivatives (Q) in MeCN have also been examined. Perchloric acid shows both accelerating and retarding effects on the hydride-transfer reactions depending on the p-benzoquinone derivative. It is shown that AcH2 forms a 1 : 1 complex with HClO4 and the protonated species AcH3+ is inactive for the reduction of p-benzoquinone derivatives. Thus, for p-benzoquinone derivatives being weak oxidants such as p-benzoquinone which have the redox potentials E°( Q Qxxx) < 0, HClO4 shows an overall accelerating effect on the reduction which proceeds through the acid-catalysed reaction of free AcH2 with Q, while for those being the stronger oxidantssuch as 2, 3-dichloro-5,6-dicyano-p-benzoquinone which have E°( Q Qxxx) > 0, HClO4 shows an overall retarding effect on the reduction where HClO4 has hardly catalysed the reaction of free AcH2 with Q. A proposed reaction mechanism involving electron transfer fron AcH2 to Q followed by proton transfer from AcH2+ to Qxxx in the rate-determining step of the hydride-transfer reactions provides a quantitative evaluation of the single and unified correlation of the logarithm of the rate constant for the hydride-transfer reactions from free AcH2 to Q with the redox potential of Q in the absence and presence of MgClO42 or HClO4. The electronic substituent effects on the rate of acid-catalysed reduction of aromatic aldehydes by AcH2 are shown to be very small, as being compatible with those observed for liver alcohol dehydrogenase (LADH)-catalysed reduction of the corresponding aldehydes by NADH. Moreover, the absolute value of the rate constant for the hydride-transfer step from free AcH2 to the protonated benzaldehyde in our model system is also shown to be compatible with that in the LADH-enzyme system.