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Let $F$ be an affine flat group scheme over a commutative ring $R$, and $S$an $F$-algebra (an $R$-algebra on which $F$ acts). We define an equivariantanalogue $Q_F(S)$ of the total ring of fractions $Q(S)$ of $S$. It is thelargest $F$-algebra $T$ such that $S\subset T\subset Q(S)$, and $S$ is an$F$-subalgebra of $T$. We study some basic properties.  Utilizing this machinery, we give some new criteria for factoriality (UFDproperty) of (semi-)invariant subrings under the action of algebraic groups,generalizing a result of Popov. We also prove some variations of classicalresults on factoriality of (semi-)invariant subrings. Some results over analgebraically closed base field are generalized to those over an arbitrary basefield.

Equivariant total ring of fractions and factoriality of rings generated by semiinvariants