The concentration dependence of single-chain equilibrium properties such as the radius of gyration RG, the hydrodynamic radius RH, and the static structure factor Ss(q) of a labeled chain in a dilute solution is due, in the first order in concentration, to the deformation of the labeled chain, both in size and in shape, when it experiences a binary encounter. We calculate the concentration correction in RG, RH, and Ss(q) using two-chain Monte Carlo calculations of Olaj et al. to quantify the intramolecular deformations in a pair. The magnitude of the correction turns out to be less than a few percent. The concentration dependence of the static structure factor S(q;C), including the interference effects, is also calculated using the pair distribution function obtained in the above Monte Carlo calculations, without resorting to the single-contact approximation for the excluded volume interaction between two chains. The results for S1(q;C) are compared with the Zimm plot. The initial slope of S1(q;C) as a function of q2 in the good solvent limit is found to depend on concentration appreciably while it is known to be independent of concentration in the single-contact approximation. Introduction The main purpose of this paper is to investigate the concentration dependence of single-chain equilibrium properties in the lowest order in concentration. Specifically, we consider a labeled chain in the presence of other identical chains in a dilute solution. A single-chain property is defined in general as a function Z ( S ) , which depends on the positions S = (SI, S2, ..., SN] of the monomers of the labeled chain relative to its center of mass. Other internal variables such as the bond vectors b = (bl, b2, ..., bN-l) could also be used to specify 2. The equilibrium average of Z is defined as ( Z ) , ( S ; C ) = I d s z ( S ) + ( S ; C ) (1) where + ( S ; C ) is the equilibrium monomer distribution of the labeled chain in the presence of others at a specific concentration C (C being the number of molecules per unit volume). The concentration dependence of +(S;C) , in the lowest order in polymer concentration, can be displayed as1 + ( S ; C ) = + ( S ) + C S d 3 R G(R)[+(SIR) +(S)l (2) where +(S) and +(SIR) denote, respectively, the intramolecular distribution of the labeled chain when it is isolated and when it is forming a pair with a center-of-mass separation distance R. Here, G(R) is the pair distribution function of polymers in the infinite dilution limit and is related to the intermolecular interaction potential U(R) by G(R) = exp[-U(R)/kd'l (3) kBT being the temperature in energy units. of (2) in (1) as The equilibrium average of Z can be written by the use G ) $ ( S ; C ) = G ) , ( S ) + CSd3R G(R)[(Z),(S,R) (Z),(S)I