Let X be a ruled surface over a nonsingular curve C of genus $$g\ge 0.$$
g
≥
0
.
Let $$M_H:=M_{X,H}(2;c_1,c_2)$$
M
H
:
=
M
X
,
H
(
2
;
c
1
,
c
2
)
be the moduli space of H-stable rank 2 vector bundles E on X with fixed Chern classes $$c_i:=c_i(E)$$
c
i
:
=
c
i
(
E
)
for $$i=1,2.$$
i
=
1
,
2
.
The main goal of this paper is to contribute to a better understanding of the geometry of the moduli space $$M_H$$
M
H
in terms of its Brill–Noether locus $$W_H^k(2;c_1,c_2),$$
W
H
k
(
2
;
c
1
,
c
2
)
,
whose points correspond to stable vector bundles in $$M_H$$
M
H
having at least k independent sections. We deal with the non-emptiness of this Brill–Noether locus, getting in most of the cases sharp bounds for the values of k such that $$W_H^k(2;c_1,c_2)$$
W
H
k
(
2
;
c
1
,
c
2
)
is non-empty.