The kinetic and spectroscopic properties of the secondary electron acceptor A1 were determined by flash absorption spectroscopy at room and cryogenic temperatures in a Photosystem I (PS I) core devoid of the iron-sulfur clusters Fx, FB and FA. It was shown earlier (Warren, EV., Golbeck, J.H. and Warden, J.T. (1993) Biochemistry 32: 849-857) that the majority of the flash-induced absorbance increase at 820 nm, reflecting formation of P700 +, decays with a tl/2 of 10//s due to charge recombination between P700 + and A~-. Following A~-directly around 380 nm, where absorbance changes due to the formation of P700 + are negligible, two major decay components were resolved in this study with tl/2 of ,,~ 10/zs and 110 #s at an amplitude ratio of ,,~ 2.5:1. The difference spectra between 340 and 490 nm of the two kinetic phases are highly similar, showing absorbance increases from 340 to 400 nm characteristic of the one-electron reduction of the phylloquinone Al. When measured at 10 K, the flash-induced absorbance changes around 380 nm can be fitted with two decay phases of t,/2 '-~ 15/~s and 150 #s at an amplitude ratio of ,,~ 1:1. The difference spectra of both kinetic phases from 340 to 400 nm are similar to those determined at 298 K and are therefore atllibuted to charge recombination in the pair P700 + A~-. These results indicate that the backreaction between P700 + and A~-is multiphasic when Fx, FB and FA are removed, and only slightly temperature dependent in the range of 298 K to 10 K,