diff --git a/scripts/ci/install-deps.sh b/scripts/ci/install-deps.sh
index bbda74ef9d..fcbf6b1198 100755
--- a/scripts/ci/install-deps.sh
+++ b/scripts/ci/install-deps.sh
@@ -6,8 +6,8 @@ scripts/fetchdep.sh matrix-org matrix-js-sdk
 
 pushd matrix-js-sdk
 yarn link
-yarn install $@
+yarn install --pure-lockfile $@
 popd
 
 yarn link matrix-js-sdk
-yarn install $@
+yarn install --pure-lockfile $@
diff --git a/scripts/ci/layered.sh b/scripts/ci/layered.sh
index 039f90c7df..2e163456fe 100755
--- a/scripts/ci/layered.sh
+++ b/scripts/ci/layered.sh
@@ -13,13 +13,13 @@
 scripts/fetchdep.sh matrix-org matrix-js-sdk
 pushd matrix-js-sdk
 yarn link
-yarn install
+yarn install --pure-lockfile
 popd
 
 # Now set up the react-sdk
 yarn link matrix-js-sdk
 yarn link
-yarn install
+yarn install --pure-lockfile
 yarn reskindex
 
 # Finally, set up element-web
@@ -27,6 +27,6 @@ scripts/fetchdep.sh vector-im element-web
 pushd element-web
 yarn link matrix-js-sdk
 yarn link matrix-react-sdk
-yarn install
+yarn install --pure-lockfile
 yarn build:res
 popd
diff --git a/test/end-to-end-tests/element/install.sh b/test/end-to-end-tests/element/install.sh
index e38f795df1..e1df709c68 100755
--- a/test/end-to-end-tests/element/install.sh
+++ b/test/end-to-end-tests/element/install.sh
@@ -12,5 +12,5 @@ unzip -q element.zip
 rm element.zip
 mv element-web-${ELEMENT_BRANCH} element-web
 cd element-web
-yarn install
+yarn install --pure-lockfile
 yarn run build
diff --git a/test/end-to-end-tests/install.sh b/test/end-to-end-tests/install.sh
index bbe7a24c9b..b2254a4ea7 100755
--- a/test/end-to-end-tests/install.sh
+++ b/test/end-to-end-tests/install.sh
@@ -4,4 +4,4 @@ set -e
 ./synapse/install.sh
 # local testing doesn't need a Element fetched from master,
 # so not installing that by default
-yarn install
+yarn install --pure-lockfile