diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index afaac5c5..96d931e2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -19,6 +19,8 @@ jobs:
       TARGET_DIR: ./target
       # Emit backtraces on panics.
       RUST_BACKTRACE: 1
+      # Apparently needed to use a2x on macOS.
+      XML_CATALOG_FILES: /usr/local/etc/xml/catalog
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index becee69f..0db0d8b8 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -75,6 +75,8 @@ jobs:
       RUST_BACKTRACE: 1
       # Build static releases with PCRE2.
       PCRE2_SYS_STATIC: 1
+      # Apparently needed to use a2x on macOS.
+      XML_CATALOG_FILES: /usr/local/etc/xml/catalog
     strategy:
       matrix:
         build: [linux, linux-arm, macos, win-msvc, win-gnu, win32-msvc]