A

$n\times n$ interval matrix

${A}^{I}={\left([{p}_{ij},{q}_{ij}]\right)}_{i,j=1,\xb7\xb7\xb7,n}$ is said to be stable [completely unstable] if all point matrices

$A\in {A}^{I}$ are stable [unstable]. If

${A}^{I}$ contains both, stable as well as unstable point matrices then

${A}^{I}$ is called of composite stability type. In the first of the two papers, conditions are established that are sufficient for

${A}^{I}$ to be stable, to be unstable, and to be of composite type. - The second paper is concerned with that special class of interval matrices

${A}^{I}$ which satisfy

${q}_{ii}<0$ $(i=1,\xb7\xb7\xb7,n)$ and

${p}_{ij}\ge 0$ $(i,j=1,\xb7\xb7\xb7,n$;

$i\ne j)$. For this special class, necessary and sufficient conditions for

${A}^{I}$ to be stable, to be unstable or to be of composite type are given.